Crossword Genius

Quite big, but not immeasurably so (8)

Ross

I believe the answer is:

sizeable

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
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