I monitored the breeding biology of House Sparrows (Passer domesticus) in a suburban colony in Cook County, Illinois, USA. I found a significant statistical correlation between clutch size and the base area of the nest box (r  =  0.592, P < 0.0029) with mean clutch size varying from 4.49 eggs (in a ‘small’ nest box, 112 cm² basal area) to 4.77 eggs (in a ‘large’ nest box, 221 cm² basal area). Other measures of breeding success (hatching and fledging success, mean egg mass, and nestling condition) had no statistically significant relationship with nest-box size. Measures of nest site preferences, as suggested by earlier date of first egg of season or by greater number of broods per season, also show no statistically significant correlation with nest-box size.