I wouldnt prune it all. Which acer is it? are you popping it in a pot or in the ground.

I agree with SBG don't prune right now the tree is in shock as it is...

I'm not sure which variety it is as there was no ID label attached.....Others nearby at the retailers were 'palmatum' I think....

Sorry I forgot to add- I think I'll transfer to a largish pot.

Don't prune!

Never prune or repot an Acer when the sap is high! You must wait until it is dormant, preferably in late winter ie. end Jan/Feb. Acers shouldn't be pruned at all really, unless of course it's become straggly and in the way of something. It you want it to remain smallish, then prune the rootball, just like bonsais.'