From what I've been reading here, apparently 1/2" or 3/4" shorter outboard is plenty enough difference. I know some of the old-time designs used 2 or 3" difference, maybe more on some. I never did like the look of such an unbalanced wing.
I personally use equal-length wings, and more tip weight as needed. Keeps me out of the controversy.
If any of the gurus here contradict my advice, my advice is to follow their advice, as any of them knows reams more than me.
--Ray