I found found many flowers
My sister also found many flowers
which one iscorrect
But mine were bigger
But mines were bigger
But mine was bigger
Does need to agree with the verb
"But mine were bigger" is correct. The other two are wrong.
Here, the word 'mine' refers to the plural word 'flowers', so you need a plural verb (were).
You cannot add 's' to the possessive pronoun 'mine'. Possessive pronouns can refer to either singular or plural nouns.