16GB should be more than sufficient, however I see only 8, 32 and 64 as storage choices on the Apple site :)
For example, I have one FMP file with 200 records, each having multiple (as many as 60) scanned documents and photos, 25 fields and the file size is 957 MB, less that one Gig.
I agree with Lyle. On my iPhone, the FM Go app is about 45MB and my solution files around 120MB. You should have plenty of capacity left for your photos, music, etc..