My hen's just started laying and now I realize my nest boxes are to ..

I would go ahead and change them now, just put some fake or store bought eggs in them so they get the idea that it is still a safe place to lay.

 

I thought 12" x 12" was an ok size and you're not to far off

 

Are you sure they are crammed in there? They are suppose to be snug so the hen feels safe.
Mine are 12x12 and I have seen 2 big orpington hens squeeze themselves in there and then a third climb on top of them.
An inch should not make any difference.

 

I would wait until they are in the run or free ranging ( if you let them) and do it then dont disturb during peak laying periods! Thats when I clean the coop and nest boxes after they have given me six or seven eggs for the day!

 

mine is 2' deep and they still back up till their tall is pushed against the back to lay.. you are probably ok

 

Well it depends on the size of your hens, my boxes are 15" x 15" x 15" and the hens are sexlinks (golden comets).
You could make new ones or possibly make a large communal box and see how they react.