Most of Minecraft's 3D models are Cubes, and so most of the textures you'll be creating are going to be squares, In order to start your texturing you'll need to choose a resolution.
Resolutions have to be a multiple of 16, as that is the base size given by the default Minecraft resource pack, you could go with:
16x16
32x32
64x64
128x128
256x256
512x512
all the way up to 2048, however it is not wise to make a pack over 256 as the game cannot handle it very well unless you have a really powerful computer, and the higher the resolution the pack is the more effort it'll take your computer to run it with shaders and mods.