Abutilon potting on

I have just treated myself to an Abutilon which is currently in a 20cm pot however it is not looking very happy at the moment so I want to repot. What size pot should I go up to please? And how do I get it looking fit and well?
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Any advise please?
Thanks for reply Philippa. I only bought this about ten days ago and it had lots of buds on but they do not seem to be opening and it just looks miserable.
I thought I would try to pot on but am not sure what type of soil to use.