Difference between 1206, 0805 and 0603 SMD resistor
The codes you are referring to are size codes, e.g. 1206 means 0.125x0.060 inches.

There are some other differences, except for size.

Smaller resistors probably feature a lower power and voltage rating, but can have smaller parasitic components, ESL and parallel capacitance. I would not expect the parasitics to be an issue, just be sure to get one with the required power rating.

If you need to hand solder them, I suggest not going too small,e specially if you do not have any experience. 0603 should be fine for almost everyone, 0805 even more so, 1206 is a huge beast you can solder with your hands tied and your eyes closed.

As usual, if you have doubts always refer to the manufacturer datasheet.