Sizing SMD components for Hobbyist Kits
0603 isn't too bad to solder by hand (I won't do 0402 or smaller though).

SOT23 is probably a good guideline (for diodes too, not just transistors); there are some SOT323s that are smaller that are a pain.

I would avoid certain SOT23-6 parts because it can be very difficult to determine which way the package is supposed to go. (For some dual MOSFET packages it doesn't matter.) We had one where there was a slight bevel along one edge. Grrr.

I would also avoid SOD123 because of the backwards nature, if possible. SMA/SMB/SMC aren't as much of a problem.

And avoid those cylindrical diodes (LL-34 / MELF) like the plague! they will roll off the board.