You cannot easily use raw photoshop files - the have an awful lot of proprietary information in them- this is private to adobe - abode creates a hidden layer just for preview in the file but transparency is not preserved in the preview layer-

convert to tif or png-

with transparency

for image size safety with graphics cards start with 2048x2048- and you need to test if you can do bigger- older machines cannot do bigger than this-
recent versions of catalyst will support up to 4096x2048 - but whether this works depends on your machines

also big files can take an awful long time to load.