I have a pizza that measures 36 by 21 that needs to cut into equal pieces with no left overs. What is the length of longest side?

I assume you mean the longest side of the pieces. Since 36 and 21 are both multiples of 3, I'd say that the pieces are

12x7

Of course, you could say that you cut it into 21 pieces each 1x36. More like bread sticks.

There's a lot unsaid about the number of pieces, and even whether they have sides of integer length. If you're going to post a vague question, you will get a vague answer.