Many time it happens we were wrong with the Estimation of Table Size, wrong estimation of Row Size can trouble us with the growth of the database and ultimately we are ending up with the space crunch, and all this because of wrong Estimation of Table Size. So what we should consider the most with Estimating Table Sizing!!! Nullbitmap is the most important criteria here to take into consideration whenever you do a Estimation of Row Size, here is an excellent article on Row Size Estimation http://msdn2.microsoft.com/en-us/library/aa933068(SQL.80).aspx and http://msdn.microsoft.com/en-us/library/ms175991.aspx
