As Badger said the size of the tire will not effect the length of chain you need - but a taller tire will effect "gearing" making it taller so you may need to go larger on the rear sprocket or smaller on the front one. Good luck!
Long enough to go around the countershaft sprocket and back to the rear sprocket and back to the beginning of the chain. Longer is not practical and shorter will make your bike a garage queen.
Next question is how long is a piece of string?
Since the chain wraps around a fraction more than 180 degrees of the rear sprocket logic tells us that when one changes the number of rear sprocket teeth by “n” teeth the safe change in chain length to accommodate this should have (n/2)+1 more link openings for a larger rear sprocket.teeth (or (n/2)-1 for a smaller diameter rear sprocket). Since a chain link covers two teeth the change in # of links is ((n/2)+1)/2 to be safe for a larger rear sproket. If an ordered chain is too long one link may be removed after purchase.Since our 480 chain link length when new is 1.00 inches/link one can easily convert number of links into a length in inches. Similarly while I forget the service limit I believe is about 3/10ths of a percent permissible elongation so simply measuring length of chain can indicate a wear limit. i.e. if 100 links stretched tight measures more than 100.3 inches it is time to replace.
Losing most of the possible adjustment is a problem when going to a bigger tire on a stock swing arm. I knew one guy that had to use a half link to make things fit.
As for measuring wear I have used .006" per rivet as the max. Counting the first rivet as zero, there are 36 rivets in 18" of chain. 36 X .006 = .216" So, if you measure what is supposed to be 18" of chain and it measures out to be just shy of 18 1/4" it is time for a new chain.