I’ve been told the rule for using kunyomi vs onyomi is that when there’s one kanji with hiragana, you use the kunyomi reading. The two words above both use one kanji coupled with two hiragana, yet “great” uses the onyomi reading whereas “size” uses the kunyomi reading. What happened to the rule when the word “great” uses onyomi instead of kunyomi?
It’s a rule of thumb, not an absolute rule. It holds true often, but not 100% of the time.
In this case, the した etymologically comes from する, and する attached to something is quite different from the okurigana of something like 大きさ. That alone kind of lessens the applicability of the “kunyomi for attached hiragana rule”.