Wheel & Tire Sizes

[planebuilderbill]

'68 Beetle with stock fenders. Air Kewld 4" narrowed Beam, 2.5" dropped spindles and Air Kewld discs F & R. What are the largest diameter and width rims and tires that can safely be used for the street??? I know this is not a particularily simple answer, but I'd like some reference point(s) to work from....

Thanks in advance....