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.

I would recommend using a SMD rework tool for small projects, and a reflow oven (you can make one from a toaster oven) for larger boards.

Reflow makes sense because you have far less problems with solder bridges and it's actually harder to destroy components. Components tend to pull themselves into position, so placement of smaller components becomes less critical (than with soldering by hand). 0805 and 0603 are a breeze.

For reflow it makes sense if you have a tool to deposit accurate amounts of solder. Using a syringe by hand and really a bad idea. The smaller the component the more critical this is.

As far as my skills, to add a point of data. Using a $40 soldering iron, and lots of flux (I have a 'pen' with the liquid kind inside), and the occasional desoldering braid.

Easy: 0805 passives , 0.7mm pitch ICs

Doable if careful, but have ruined a couple: 0603, 0.5mm pitch ICs

Haven't tried smaller than those yet, I think that's about my limit.