The problem with faceting with size is that the ordering by size is not directly visible. Let us take the following sizes:

XS, S, M, L, XL, XXL

These sizes would be listed in the alphabetical order as follows:

M, L, S, XL, XS, XXL

To handle such ordering scenarios in size for different apparel, we could encode a size tag into the size facet label. Therefore, the size ordering would be somewhat as follows:

[00002]XS
[00003]S
[00004]M
[00005]L
[00006]XL
[00007]XXL

This will ensure that the facets we get from Solr are ordered in the way we want them to be ordered.