I never had the RC version, but it's probably near the same size as the CL venturi. Three ounces of fuel is probably more than enough. So if you've got a 3 oz tank, use it and reduce fuel if you're going over the time limit.

For an OS Max 25?  That's a plain old OS Max 'nothin' 25, not LA or FA or Slartibartfast or anything.  Venturi size is "the RC carb wired open".

I've got an old Perfect wedge tank that's 1 x 1.75 x 3, which I'm estimating at around 2.5 - 3 ounces, but I suspect it'll be too small for a full pattern.

(I'm putting together a practice/backup/'dangit I broke my real plane' plane).

    3 ounces is probably enough if you run it normally. Depending on what airplane it's in, you may want to pump it up (nitro, etc) and then you might be in a bit of trouble with 3 oz.

    I note that the 25 we are talking about putting in full-size Noblers, Twisters, etc. is not the motor you are describing. I have a motor like yours (25-S Baffle piston from the 70's) and while it's a nice runner it won't fly typical 35-sized models very well. A Still Stuka is about the max and that won't have a lot of margin with conventional fuel. A light Ringmaster would be OK.  I would also note that I broke the crank in my engine running it on Cox Racing fuel to get more ponies out of it.

   The last airplane I flew with mine was a Jr. Nobler, and that had *a lot* of margin. Until I crashed it trying to do a reverse wingover. That's not an ideal combination either - if I had another Jr.Nobler I would use either a Cox Sportsman 15 (I have *two* of those), a Max III/II 15, or an LA15 (because I know how to handle the grossly excessive power). A 15LA has considerably more power than a 25-S.

     I am curious - what sort of airplane are you planning to fly with a 25S? There aren't a lot of airplanes in the right size range.

     Brett