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:
XS S M L XL XXL
This will ensure that the facets we get from Solr are ordered in the way we want them to be ordered.