The trick with little screws seems to be to exactly as possible fit the slot in both dimensions, so you can end up with a LOT of screwdrivers if your serious.
I have a wiha set down to T3 but have not had use for it yet. I think I read somewhere recently that someone is offering a T1.Hex and Torx have a finite number, smallest Torx is what about a T3, and hex has maybe 3 or so sizes below 1 mm?
Small blade screwdrivers not only have different widths, but different thicknesses. In watch repair they sell sets to match various brands of watch, apparently the oddball sizes are at least standardized within brands.
STARRETT jewelers screw drivers NO.s555-6 set of 6 is as good as youll ever put your paws on, the set is .025-.100
http://catalog.starrett.com/catalog/catalog/groupf.asp?groupid=240