I have flown 1 IMAC contest in the SW region. I flew my 60-90 Edge. I had the smallest plane on the flight line but I finished above some of the guys flying 50CC size airplanes. Two of the guys in basic were flying 100 CC sized airplanes. A young man 14 or 15 years old, flying a 1.20 size Giles finished I believe in second place. It is possible to fly a small airplane and be competitive. Remember it's not the size that matters, it's how you stir the sticks....
It never hurts to have your girlfriend in a very skimpy bikini calling for you as well
It distracts the judges and since if there is any question on a maneuver, judges are required to give you the benefit of the doubt, well you see where I'm going with this
All kidding aside, basic is the come and fly what you got. Is it possible you'll be flying against 50cc and larger stuff, you bet but I've seen guys will 90 size Ultrastick's kick major butt because they understood exactly what needed to be done, to the time to setup the airplane properly and and learned how to properly flew the sequence
If at all possible, get someone close to your area to sit down and watch you fly. Then listen to what they have too say and do what they tell you. IMAC is loaded with a bunch of great people that are more than willing to help you be the best you can be and if you really want to learn go to a contest and do nothing but scribe for the judges. Most judges will give you the reasons for the scores they hand out if you ask, time permitting. They will see things that look fine to you at first but in short order you will start seeing little things and since we are talking Basic, the first maneuver is a 45 degree upline, you will learn how to actually tell if the pilot was at a 45 degree angle on not 35 or 55.
And one last thing, try not to be embarrassed, as IMAC pilots we do remember that we were all there at one point or another. No one is gonna make fun of you and there is no shame in starting in Basic. I've seen tons of excellent 3D pilots start out in Sportsman or higher because they felt basic aerobatic maneuvers were below their own standards only to have their butts handed to them when the scoring started. Even if it's one or two contests do basic first to get a feel for IMAC and if you are feeling challenged enough, then move up to sportsman