Function size

size(expression: Expression, library: ImageLibrary): string
Compute the size of an image meme expression.
Parameters
- expression: Expression
  the expression to use
- library: ImageLibrary
  the library of named image files
Returns string
"WxH" (in the same syntax as the corresponding part of the @ resize operator) representing the width W and height H of the image generated by the expression, according to the language definition in the PS3 handout

Throws
Error if the size cannot be computed