I'd say, OS 20fp, APC 9x4 revved out, and back it down enough to keep it safe. 60' .015 lines.
I had 12's break, but if you're careful with them, it's not a big deal I guess. I think people would like the feeling of control difference between 12's and 15's though. Like a more solid feel.
I've asked the question about line lengths also. It kinda (kinda,in that this isn't by any means a rule to remember) boils down to, pick a known line length and trim your airplane to suit. So if you can fly all the manuevers you can fly on that set of lines, then they're correct. Until you get pretty good then experimentation yields results for things we don't know how to look for at our beginning stages.
I tell everyone to start with the 20FP/25LA with about .015x60-62', because that will be short enough to have line tension and moderate effect on the yaw angle (in case the leadouts are misplaced or not adjustable), and not too short to screw you into the ground when the engine is set correctly.
The biggest issue, by far, is managing the speed *without* resorting to needling (or attempting to needle) the engine down excessively. FAR too many people equate "2-stroke" = "meltdown" (which it could be in some circumstances with vintage engines), and are afraid to run the engine like it needs to - medium 2-stroke in level flight.
You can recognize them from on-line posts where they talk about 52' lines, "what happens when the wind blows!?" and "it just runs away!". "Running away" is the way it is *supposed to run*, and if you don't run it that way, you will not get the results that everybody else gets. Set the 20FP/APC 9-4 properly, on a Ringmaster, and I guarantee that wind will be the last problem you have. The wing is an inch thick and you are going 75 mph - wind is not an issue!
To re-iterate, set the engine (20FP/9-4 APC) by peaking the needle on the ground, that is, as lean as it will go before it starts to sag, then back off richer until you get a *distinct* drop in speed, maybe 5-6 clicks. It should wind up in a medium 2-stroke in level flight, and speed up in the maneuvers. If it ever goes "over the top" lean in the maneuvers (like at the end of the flight), reset the needle 1 click richer on the next flight, until it doesn't. If it ever does more than an occasional misfire into 4, set it one click leaner on the next flight, so it never goes into a 4-stroke, or it goes "over the top" lean.
Note that I am not giving RPM readings because I don't want you to go by RPM, go by the sound and how it works in-flight. A 20FP will usually be somewhere between 11,500-12,200 on the ground, at sea level, on 10% fuel, but that's not a goal to shoot for. Get it working, THEN tach it to see what it is doing. The "new" 25LA will be substantially faster than that to get the same level flight speed.
Brett
p.s. again, .012 lines are OK if they are in good shape - but you are flying a 30-ish ounce airplane at up to 75-80 mph, it's not going to pull like a 1/2A. If someone already has those, keep going, but be very picky about the condition of the lines and discard them if they have *any* apparent issues.