First pedal bike, what size wheel?

[WindUpBird]

I'd like to get DS a pedal bike for his 4th birthday. He already has a balance bike with 12" wheels, so should I go for a pedal bike with 12" or 14" wheels? I don't want it to be too difficult/heavy for him, but equally would like it to last a year or so till he's big enough for DD's 16" wheel bike! I'm hoping to get a secondhand one.

Depends how tall he is!
Take him to a good bike shop (not Halfords or a toy shop) and get them to advise, then look second hand. They need to be able to out both feet flat on the floor for a balance bike, but only just for a 'real bike'.
He should be off and away very quickly without using stabilisers if he is good on his balance bike.
DS had a 12" til he was 6, but is on the small side.

I know, I guess it was a silly question! Well, I've managed to get a 14" second hand so if it's too huge I've still got time to find a 12". He's really good on a balance bike so I'm hoping he'll take to it really quickly...