Relative size has girl worried (7)
I believe the answer is:
largish
'relative size' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?
'has girl worried' is the wordplay.
'worried' indicates an anagram.
'has'+'girl'='hasgirl'
'hasgirl' is an anagram of 'LARGISH'.
Can you help me to learn more?
(Other definitions for largish that I've seen before include "Of considerable extent" , "quite generous" , "Fairly big" , "Somewhat hefty" , "sizeable proportions" .)