Size Matters sucks compared to every other game in the series. Maybe because it's no Insomniac.
I do, however, recommend Syphon Filter: Dark Mirror. It's my one good PSP game and one of the only shooters that works on the PSP.
Oh and Syphon Filter: Logan's Shadow. I wish they'd stop wasting time and money on these PSP games and make a proper PS3 title (with the quality of the original Syphon Filter, nothing like Omega Strain please)