I’m posting this so I have my own record of it. I always have to look it up and this is a great place to store my ‘notes’.
Typically, tire sizes for a R75/5 R90/6 R90s R100S, etc will have a:
3.25 x 19 Front
4.00 x 18 Rear
The metric metric equivalents would be 90/90-19 for the front and 100/90-18 or possibly 110/90-18 for the rear.