Aim for a 300MB to 450MB CSO file. That is the sweet spot for the "Highly Compressed" experience.
Because the PSP’s CPU had to decompress data on the fly, heavily compressed CSO files often resulted in stuttering gameplay or significantly longer loading screens between matches. fifa 11 psp highly compressed
If you cannot find a stable link for FIFA 11, these PSP football games are just as good and often have smaller file sizes: Aim for a 300MB to 450MB CSO file