You need to consider what size you can fit inside your wheels firstly, then scour parts list to see if another car uses the disc size you want with a similar offset(height).
You can't just go buying bits and assuming they can be fitted and made to work, if the offset was wrong it could foul on the inside of the wheels for example on the spokes, if it had a much higher offset(height) would put it closer to the spokes of the wheel.
If you are starting from scratch then you can buy the closest thing that you need, may(very likely) even be able to find a disc from another ford or other make that suits your needs and just redrill it to your pcd, totally safe and been done loads of times.
You can also use other makes of caliper, ie mb/porsche/evo 4 pots etc with a bracket.
Your car should have 300mm with a height of 47mm, they are a bit thin at 24mm(new) for a 300mm disc, but phone ebc and get a application book, then scour it, is the best way, for example the 5.0 mustang cobra uses 330mm height of 53 but 28mm thick, which I very much doubt your calipers would accomodate.
The best way I find is decide which calipers you want to use first 4 pots will give better braking but cost, but a upgrade to say 325mm/330mm would be noticeable but not great, I don't have the dims on the mk2 FRS but I wouldn't be surprsid if these were bigger and could be pursuaded to go on.
tabetha