[Hall of Famer]

I dont think the size/dimension really matters, it's more about consistency. If your site uses sprites that are 16x16, just stick to that standard and dont use 32x32 or 48x48 sprites interchangeably. For me the ideal dimension is around 48x48 or 64x64, but there' no universal standard so you should be fine unless your sprite is too small for your members to actually see what it is.