Determines the dimensions (8)
I believe the answer is:
measures
'determines the dimensions' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for measures that I've seen before include "Ascertains the size, of dances?" , "Evaluates" , "Establishes size, mass, etc" , "Finds the length of" , "Sizes up" .)