The size of the pressure plate and clutch disc are to be whatever the flywheel is drilled for currently. The only way to change the size would be to have the flywheel re-drilled for a different diameter pressure plate, or to get an entirely different flywheel. Any size will work fine in a relatively stock application. The throwout bearing should be fine as is. What to use for a disc may take some work depending on the splines used on the tranny, but I would start to look for a disc based on year and application of tranny, not the truck or engine. Yes, any good parts store should have the parts, and should be able to help sort out the disc issue. Otherwise, try the specialty vendors that litter the internet and Hemmings Motor News.
Sounds like you are running 5 on 4 1/2" bolt pattern then?
T5 doesn't use rings or notches. They do have a code sticker on the top cover plate that can be cross referenced for identification. Or you can just mark the input and output shafts, put it into gear, and turn them to see what the ratio is.
