How are aircraft size classes defined?
I would believe that you are talking about Aircraft Design Group classification which is based off of aircraft wingspan and tail height. These classifications can also dictate what taxiways a certain model of airplane can use. 


Group I: Wingspan - Less than 49' (15m)
Group II: Wingspan - Between 49' (15m) and 79' (24m)
Group III: Wingspan - Between 79' (24m)  and 118' (36m)
Group IV: Wingspan - Between 118' (36m) and 171' (52m)
Group V: Wingspan - Between 171' (52m)  and 214' (65m)
Group VI: Wingspan - Between 214' (65m)  and 262' (80m)


PDF for list of Boeing Airplane Group classifications