If you don't have any special demands for a laptop beyond checking email, paying bills, or browsing the web, any size will do. You should probably stick to 13 or 14 inches, since anything larger might ...
Most of the computers from Apple's lineup have already upgraded to the M4, and only the MacBook Air remains with the M3 as its most advanced model in the market, but that is set to change real soon.
You can now get your hands on a refurbished 13 inch MacBook Pro for just £159 - but you'll need to be quick as the deal is only available for a limited time ...