I would say just let the bladder expand to fill the tube.
Mainly because pretty much every model I have ever had worked that way

If the tube is too small it will create "false pressure" when the fuel is forced into the bladder during filling.  Soon the fuel will be depleted to where the bladder is loose, and a drop in pressure will make the engine go lean.

So size the tube to where it is slightly snug, but not constrained.  If it takes extra effort to complete the fill, you are probably making false pressure.

That's along the lines of what I was thinking. I imagine the most consistent pressure from the bladder would require it to be allowed to grow to it's full expanded shape.
So for this smaller wing (the Jr. Satan), I think I may go with latex tubing that will fit when fully filled.

Also, don't ignore pacifiers or pipette tops.  They are easier to make than tubing bladders and they have lower pressure, which makes it easier to needle the engine.

Can you provide a picture or an example? This is the first I've heard of using pipette bulbs for bladders.
And I just read somewhere that pacifiers provide higher pressure than the latex tubes.   

I have been having the same problem, I have Nemesis II models that have really inconsistent runs and go extremely lean and then die with a fair amount of fuel still in the bladder.  The bladder tube is a Estes bt55 rocket tube that is about 1 1/4 id and squished to a slightly oval shape in the model.

Here's what I'm learning -
A 3/16" OD, 1/8" ID latex tube fills up to right around 4" long with about 1.8 oz of water (fuel). See the pic. The inflated diameter actually comes to about 1.1".
The clear bladder tube provided in the Jr. Satan kit is 1.2", so the bladder won't be constrained.
I measured/modeled a simulated tank so that the length is 3 1/4" long (to where the bubble necks down to the un-inflated diameter) (in CAD).
At 3 1/4", it will fit in the 4" long kit tube, and the volume works out to just a tad over 1.5".
That works for me ... Should get 4 mins on that with a stock 15 engine (LA/FP etc).

I previously bought the 3/16" OD latex tubing from Terminal Tackle, but they're gone.
I found another source - Jann's Netcraft; $6.35 for 5ft., a few several colors available.

And, of course, it took me 12 attempts to post the pics because, little did I know, but a 700 kB picture is actually GREATER than the 1000 kB limit. Reverse Polish Math - I get it.

Keith- some good replies here.  In addition:
The bladder pressure depends on the i.d. of the compartment, the wall thickness of the bladder, whether it rests against the wall. and how it was trained.
F2D planes seem to mostly use a 1.5in(~38mm) tube about 3.5in usable length.  That will hold enough fuel for a 4+min. run and usually a bit under 2min. for Fast combat, depending on nitro content.  Mostly the bladder seems to be trained to blow up in the middle and fill towards the ends. It's important not to overfill it or you get the false pressure.  You can train it to blow up from the closed end.  Then it's important to make sure the closed end goes into the corner at the far end of the bladder tube.  Otherwise it may run into the end and give false pressure  or into the wall cutting capacity.  For F2D planes you need all the fuel you can get.  Chop up a Sullivan Crap Trap filter for the aluminum end pieces.  They make very short ends that don't take up too much space.

If you need to use a smaller 1.25in. tube it needs to be about 8in. long for Fast combat. 6.5in. or so works well for smaller engines in Speed Limit or similar events.  Cut a slot in the sheeting or foam so you can see what the bladder is doing.  In either case, lubricate the bladder with silicone grease.  Train it to blow up from the closed end.  You may need to pull on the fuel line to keep the bladder centered in the tube. HOLD onto the fuel line at the syringe unless you want a bath in fuel.(applies above too).  Always force the bladder to start a filling at the closed end before you put it in the plane for the first flight of the day.

Either method can be used with Half A if you scale down to 1/32in. wall bladder tubing- usually 3/16 or 1/4 i.d. and 1/32in. wall.  The bigger tubing will give a little lower pressure.  If you're buying ARF planes adjust the diameter and length to suit.

Another "gotcha"-  latex rubber, virtually the only thing available for flying, is very sensitive to saliva.  If you want to see a piece of rubber fuel go south lick the end to get it onto a fitting.  Within a couple of days it will start getting soft and sticky. IF it DOESN'T it isn't latex.  There are several alternatives including neoprene, Norprene, reddish silcone tubing(stronger), or clear/pigmented(not as strong).  Same happens if you get spit on a bladder.  When it starts to get sticky throw it away and get some silicone grease for lubing rubber(fuel line and bladders!)

Latex is also very sensitive to UV(ultraviolet) in direct sunlight.  Get tubing with a colored layer.  It helps block UV and it telegraphs failure when you start to see pinholes in the color layer.  After the first pinhole you may get another flight, after that you're on borrowed time.

McMaster-Carr has about 100 different varieties in multiple sizes.  Search for flexible tubing.
I just ordered some of this:High-Temperature Silicone Rubber Tubing for Air and Water
Soft, Durometer 50A,
1/16" ID, 3/16" OD,
Semi-Clear Blue, 50 ft. length
5236K207    Item Number
We got clear/misty tubing last year.  The blue tubing is identical just a pretty color!


It's been awhile, but I ran about 50 pressure tests on various bladder set-ups.
The old Binky pacifier(Binky still advertises but I haven't tested the current version) blew up to about 6psi.  It would hold over 4oz. of fuel in a large enough compartment.  The pressure dropped only slightly until the pacifier was under an inch diameter.  Not quite enough fuel for current Fast combatengine.;

1/32in wall latex has about the same pressure.  The bladder tube should be slightly smaller(3/32 in+/-) to prevent the tubing from failing early.  Depending on the tubing, 3/4-1in., for 1/4in id x 1/32 wall.

1/4 x 1/16 wall latex blows up at around 20psi.  This requires a much finer control of fuel flow from the needle valve.  Fast combat is OK with the huge amount of fuel they suck with SuperTigre style brass/lock nut style construction.  For anything else the current best is an F2D nva.  Ones like the Fora have a very usable setting range on F2D, Speed Limit, even Half-A.
     OS stopped making the 1a remote needle valve.  It was for the early OS 10 and 15 engines, and later up to the 25's.  It has an adequately fine thread and a smaller overall size and is easy to confuse with the many other NVA's OS builds(tm).  If you can find them they offer more mounting options than F2D NVA's.  Some people prefer that, no problem.

1/4 x 3/32 tubing works in F2D.  It is very hard to blow up without using a 3/4in. diamter pump.  A 1in veterinary often used takes a LOT of muscle.  The bladder give around 30psi.  That much pressure isn't really needed.  The 3/32 wall tubing is substantially longer lived than 1/16in. wall.

There are also some tricks to bladder construction and use I'll get to later, unless somebody else does it first.