I have to agree with The Shiner.
I've used both A4 and A5 and the only difference I found was that the A5 are easier to fold to get through a letter box.
They're less expensive too. A5 is exactly half the size of an A4 sheet of paper; therefore just print (or stick) two A5 flyers onto an A4 sheet and take them to whoever is going to copy them.
They're easily cut (normally by the photocopying company), but if it's a friend photocopying them 'on the side' at work; just purchase a guilotine and cut them neatly.