Quite big, but not immeasurably so (8)
I believe the answer is:
sizeable
'quite big' is the definition.
'sizeable' can be an answer for 'big' (similar in meaning). I'm unsure of the 'quite' bit.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sizeable that I've seen before include "Large" , "Largish" .)