That pot is ample for such a tree, and probably forever! Acers are woodlanders, and as such, prefer a sheltered site, away from cold and drying winds, in a well-drained soil that doesn't dry out. But, they don't like to be wet, especially over winter. I keep mine on the drier side in the nursery situation and they're fine.

Definitely will need feeding. Use a 'balanced' feed - one which has NPK numbers of equal size. (look on container - it will have N. P. K. with a number by each) This will be good as a liquid feed, but also consider a scattering of pellet food (chicken manure etc) and put a thick mulch on top of the surface. Leaf mould is ideal, or compost or comfrey minced up, it will help retain moisture and protect the roots. I would feed it regularly and keep it mulched all the time.

If its very wet though might be best to hold the feed until the worst has dissipated, as if its under stress it might not be the best time now. You could try a foliar feed for starters.

Thanks again for your help