They often cause bother, being available in two sizes (5)
I believe the answer is:
louts
'they often cause bother' is the definition.
(a lout is an aggressive or troublesome person)
'available in two sizes' is the wordplay.
'two sizes' means two replacements for 'size'.
'available' becomes 'out' (I've seen this in another clue).
'in' indicates putting letters inside.
'size' becomes 'L' (large clothing size).
'size' becomes 'S' (small clothing size).
'l'+'s'='ls'
'out' put inside 'ls' is 'LOUTS'.
'being' is the link.
(Other definitions for louts that I've seen before include "Uncouth aggressive men" , "Dolts" , "Yobs" , "Coarse types" .)