Normally, the sanding discs are available either
normal hard, or
somewhat softer.
About the same applies to the hardness as to the size. If the surface is not yet completely flat:
softer plates reach the whole surface faster, as they adapt to the curvatures a little better
harder plates need a little more time to reach the whole surface, but then it becomes better flat.
Note: for soft wood prefer hard discs!
Recommendation: only change from hard to soft, not vice versa (a surface sanded relatively wavy with a soft wheel is difficult to level with a hard wheel).
My personal impression, however, is that there is little difference between the various hardness grades.