What size tire width can fit in an oval spare tire bay?

Small. Very small. I can't imagine that it's any bigger than a late bug, and a stock 4" wide Beetle rim with 165 tyre is tight in those.

So a 6'' Fuch would be to big?

Absolutely. There is no way you're going to fit a 6" wide rim into any Beetle spare wheel well. Most guys on Porsche stud pattern tend to buy Porsche space-saver spare wheels as these will fit into a Beetle wheel well.