4 years old girl bladder size 261cc, 3 cc postvoid is it normal size ?

Yes.: Large scale studies of children have shown that the bladder capacity of children follows a nonlinear curve that is related to age. Based on a study of 5, 000 children in boston, a 4 year old would have an estimated volume of 240cc with girls having a slightly larger volume. The 4 year old girl you describe falls within the normal and expected range. The post void is normal and healthy as well.