Mike
2.5 should be fine for OTS , But as Tom H said , a bigger tank will be even better, and more open to other fuel types. I use a FP25 on S1 Ringmaster using Powermaster 10% nitro 22% oil 1/2 syn 1/2 castor ,Run pretty much all out in a fast two stroke . Runs about 6.5 minutes , plenty for OTS, but a little tight for Full pattern . If you have room, I would opt for bigger tank ,
The more nitro you use the more fuel you will require, the more 2 - 4 break you run the more fuel you will require and on and on ..
Will