How do I use a frequency distribution to find the class width?

Simply take a class then take the difference of upper limit and lower limit and add 1.

example

Class 1 - 4 5 - 8 9 - 12

Take 1 - 4.. The right end numbers are upper class limits and the left end numbers are lower class limits.. to take the class width or class size

UL - LL + 1 = 4 - 1 + 1 = 4, therefore the class width or class size is 4

meaning there are 4 possible values from 1 - 4 those are 1,2,3,4