Tyre Sizes

Hi all. I've been looking through the various forums and have found some information about tyre sizes, but nothing which answers my question.

I need more ground clearance so need to change my wheel/tyre combination.

I realise that I can get an inch rise on the front by jacking up the torsion bars, but if I do that what is:

a: the best tyre size to maximise lift on the standard 15" rims I have?

b: the best tyre size to maximise lift if I fit 16" rims?

I don't want to modify the wheel arch if I don't need to, but happy to hammer out a little space if required. I'm not looking to fit wider tyres. This really is for off-road so still need full suspension travel and wheel Arch clearance.

Many thanks for any replies.