Why can't you say "Những con muỗi là nhỏ"
I think là is only used for nouns, like saying "he is a doctor" or "she is a mouse". For adjectives "he is tall" etc, you either leave out the verb or use thì.
Could you just drop thì altogether here?
What's the difference between thì and rất?
Thì is to me similar too là (is). I think you can ommit thì in this sentence. Rất means very or really.