Considering the size of the leaf:
1 - It depends on the tea varietal and cultivar, e.g. small leaf (sinensis) vs. big leaf (assamica) varietal. In general, puerh tends to be produced from the big leaf varietal. See here
https://teadb.org/puerh-oddities/
2 - Grade of the leaf: 0 to 9 according to the size of the whole leaf; the smallest are the buds. In case of shu the finest leaf are commonly referred as
gong ting grade. The big old leaves that are often sorted out are called
huang pian. While there is sometimes a preference for small early spring leaves, it is not true in general that smaller is better or that spring is always better then autumn. Presumably, different grades of puerh leaves may age differently, see
https://www.kyarazen.com/secrets-tea-seasons-aging/.
EDIT: another reference
https://hojotea.com/en/posts-56/
3 - In factory formulas like 7532, 7542, 7562 etc, the third digit is sometimes said to correspond to the average grade of the leaf. In my opinion, it is better to completely avoid this interpretation, and consider the whole number as a name of the tea.