| Size code | Length (mm) | Width (mm) | Height (mm) | Power (W) |
|---|---|---|---|---|
| 0201 | 0.60 | 0.30 | 0.25 | 0.05 |
| 0402 | 1.00 | 0.50 | 0.35 | varies |
| 0603 | 1.60 | 0.80 | 0.50 | 0.063 |
| 0805 | 2.00 | 1.25 | 0.50 | 0.100 |
| 1206 | 3.20 | 1.60 | 0.60 | 0.125 |
| 1210 | 3.20 | 2.60 | 0.50 | 0.250 |
| 1217 | 3.00 | 4.20 | 0.900 | 0.250 |
| 2010 | 5.00 | 2.60 | 0.70 | 0.250 |
| 2020 | 5.08 | 5.08 | 0.90 | 0.500 |
| 2045 | 5.00 | 11.50 | 0.90 | 1.000 |
| 2512 | 6.30 | 3.10 | 0.60 | 0.500 |
Source: https://www.westfloridacomponents.com/surface-mount-resistor-sizes.html
In my experience, 1206 is the smallest that you can reasonably hand solder in quantity, while 805 is the smallest that is reasonably easy on the heating pad.
Precise dimensions differ between vendors. Power dissipation may also vary.