Speaking generally, near a number five hundred? Big! (2,3,5)
I believe the answer is:
by and large
'speaking generally' is the definition.
(I've seen this before)
'near a number five hundred? big' is the wordplay.
'near' becomes 'by' (near means close by).
'number' becomes 'n' (n can represent any number).
'five hundred?' becomes 'D' (Roman numeral for e00).
'big' becomes 'large' (synonyms).
'by'+'a'+'n'+'d'+'large'='BY AND LARGE'
(Other definitions for by and large that I've seen before include "In general" , "All things considered" , "On the whole" , "Speaking generally" , "for the most part" .)