Depends on what you expext them to be able to do.
And obviously how much space you have for bigger ones.
Just by guessing, I would say 5" speakers might be good. Yamaha HS 5 are great value for money.
With 4" speakers you can always add a subwoofer if you're missing the low end too much.
Personally, I can't imagine 3" speakers to sound anywhere close to acceptable and enjoyable
Thanks ! I am really not sure which monitors should i buy, i know Adam audio have good reputation and A4V model looks good for my need, but maybe should i consider the budget Monitors (Yamaha HS 5)? because my room has no acoustic treatment