Is it Oxalis regnellii? (see link) Or something similar? It has a fleshy rhizome, so the pot should be big enough to accommodate some undergroung growth. Give it as much sun as you can (IMHO). I think it might be too cold for it still? Although the linked page says it's hardy to zone 7, I would not grow it outdoors here in a container.

Here is a link that might be useful: Picture of a purple oxalis

8" will be fine.
Actually they will expand into whatever size you put them in.
You don't need much depth though,
4-5" is fine.

Be sure to bring them in, before the first frost, and give them a Winter rest.
Just don't water and stick in a closet
(hides the ugly bare dirt).

About a month before last frost start watering and they will pop out just fine.

You can easily keep them going indoors, but they flower and multiply better if allowed to go dormant for at least a month.

Ric