The section The first 96 powers of two shows the values you are looking for. Try to stay under 2 to the 12th power, as those can also take extra time (not to convert, but to load and display).
A picture that is 256x256 if average for something small, where Icons are usually 32x32 or 64x64 in size. Putting together a sheet of different textures is good around 1024x1024. The numbers don't have to be the same size, for example you could have a sheet of 2048x512. These are sometimes useful as they can fit longer thin textures on them.