Tyre size help with 5inch wide fender

Hi guys I got a 5 inch wide rear fender that ive metal flaked and pin striped really want to use on my build in running yam xtz 17inch rear wheel just wondering whats the best tyre size to go with cheers

A 120 is 4.72". A 130 is just over 5".

25.4 x 5 = 127 mm

Your rim width is a factor to tire width.
For example, a 120/80-16 tire on a 2.75" width rim has an overall width of 4.7 inches, as J-Rod suggests.
His measurement (5") for a 130 is the number for a 130/90-16 tire on a 3" width rim.

Tire width is a nominal measurement.