I have a 16GB card and it works fine, although now struggling for space as i have a few games downloaded such as Gravity Rush, Wipeout, Everybody Golf and handful of PSP games. I would say avoid anything smaller as you would miss out on gems such as Sound Shapes.
I say get 16GB if your planning mostly retail purchases as you still have plenty of wiggle room, and get the 32Gb if you want to download a mainly.
Edited by Sharzam at 20:55:44 28-09-2012
Known as 'Sharzam' in 98.5% of games