Fender diameter depends on two things.
take your tire diameter and add two times your clearance to figure fender diameter. (1/2" minimum clearance recommended)
EXAMPLE
tire diameter = 26.5"
desired clearance = 1/2"
fender diameter = 27.5"
TIRE DIAMETER CHART
MAKE&MODEL / DIAMETER(in) / WIDTH(in)
VINTAGE
Avon MKII
4.00-18 / 26.7(in) / 4.4(in)
4.00-19 / 27.6(in) / 4.3(in)
5.00-16 / 26.3(in) / 5.2(in)
Coker Classic
4.00-18 / 25.75(in) / 3.9(in)
4.00-19 / 28(in) / 4(in)
4.50-18 / 27(in) / 4.5(in)
5.00-16 / 26.3(in) / 5(in)
Firestone Deluxe
Champion
4.00-18 / 26.6(in) / 4.25(in)
4.00-19 / 27.6(in) / 4.25(in)
4.50-18 / 27.4(in) / 4.84(in)
5.00-16 / 26.4(in) / 5.35(in)
Dunlop K70
4.50-18 / 26.6(in) / 4.3(in)
MODERN
Dunlop D401
130/90-16 / 25.4(in) / 5.1(in)
Dunlop D401
150/80-19 / 25.4(in) / 5.9(in)
Dunlop Qualifier
120/90-18 / 26.5(in) / 4.72(in)
Avon Cobra
120/70-21 / 27.6(in) / 4.72(in)
Shiinko Classic240
MT90-16 / 25.3(in) / 5(in)
Maxis M6011
Classic
130/90-16 / 25.4(in) / 5.35(in)
Kenda K657
Challenger
130/90-18 / 27(in) / 5.1(in)