For exemple:
大きさ is an okurigana because it has a Kanji followed by a hiragana, so it means that I should read as “おおきさ”(Kun’yomi). However my question begins here, the word “大した” the “大” is followed by hiragana as well but instead of reading as Kun’yomi “おお” we read as On’yomi “たい”. Why tho?
Belth is right. That is the reason in this case, but I would recommend you break down all absolutes you may have in your mind about rules of this language. There will be exceptions to this rule outside of する and there will be exceptions to everything else you learn in this language for the most part.
Actually, the “kanji with okurigana always uses kun’yomi” rule is the one exception to the “all rules have exceptions” rule, which always makes me feel warm and fuzzy.
Yeah, we don’t need to do that list battle again, but there are lots of those [onyomi]+[modified version of する that is now effectively part of the word] things.
it took me a while to figure out that す, ずる, じる, and した are all either present or historical forms of する. i remember when WK taught me 信じる and said “信 is しん no matter what” i thought it was both the kun and on, when really it doesn’t have a kun at all.
i know WK isn’t a grammar website but i think vocabs like this would benefit from just a little explanation. for me at least, deducing readings from grammar rules takes up a lot less mental space than mnemonics.