Acer quandary

Ffoxglove

Hello! I have a Japanese maple - palmatumn maybe- that's about 150cm and has been in a 45 x 35 pot for 3 years. I just bought a much bigger pot for it but I was wondering

A when to repot
B what to use interns if soil 
C is my pot too big? It's not quite twice the size if the current pot. Does size matter?

Thank you to anyone who can help! 

Fairygirl

Pot on gradually - so a couple of inches bigger.
You can repot anytime. Just follow the usual system - water well first etc, and have the new pot ready
I don't understand B

Ffoxglove

Thanks @Fairygirl I meant for B what's the best compost mix or medium for acers... Ericacious? Thanks for helping 

Ffoxglove

I've put the pots I have side by side and bit seems I've rxxagersted the largest pots size... I think it might be only a bit jigger. 
Which would you transplant into?

Ignore the magnolia it's in another pot inside the large black pot and can be moved 

Ffoxglove