I just bought a OS BGX 3500 at a deal from a friend. I thought about trying it in my SIG Extra 300 but I think it may be a little to much of it. Can someone tell me, who makes and ARF in a Extra 300 that this motor would be good on. Im a little confused on airplane sizes compared to motor use.

An 80" Extra would do pretty well - check out the GP Patty, the World Models Extra, the DP Extra and Edge, Kangke Laser, etc. A friend of mine had one in a Midwest Extra; the others I mentioned are nearly the same size and weight (if not a little lighter).

It should fit your Sig just fine. Take a look at mounting it at a 45deg angle to the fuselage. It will route the exhaust to the bottom.