This is actually easy, but it one of the most confused issues around.
IMAA: LARGE planes Mono-planes over 80in wingspan, Bipe-Planes over 60 in,, OR 1/4 scale models or larger.
IMAC: Scale planes that fly Aerobatics, can be ANY SIZE, but most nowadays are Large up to 50%. TOC planes are a example of large ones.
So if you want to fly in a IMAA fly-in and you have a 1/4 scale or larger IMAC plane you can.
If you want to fly IMAC, and you have a large plane is has to be SCALE, and have interred into an IAC (full sized aerobatics competition) box.
This really a simple explanation and does NOT cover all the fine details