You can buy almost anything in a pot and there's no law to say you can't leave it in a pot.. You will have to keep re-potting it as it becomes 'pot bound' (used up all the soil) You then will need either a bigger pot or take it out trim both the stems and the roots before putting it back. If you're wanting to take it from your garden, do the latter.

I would think it'd need a pretty large pot given the amount of growth they make each year/season.
I'd put it into a pot that was about two and a half feet deep and about two feet diameter - i have a pot that size and this plant would be perfect in that, i think it would look top-heavy and daft in anything smaller though.
Just my opinion !

I agree with Louise - and recommend you keep it shaped regularly by pruning it back.