NILM. 

Nuclear size is estimated by using intermediate cells as reference.  Reactive nuclei could reach the size of 2.5 x intermediate cell nuclei.  I usually call it ASCUS when it is >2.5x, except for older women.  Due to hormonal changes, squamous cell nuclei could reach 4-5x the size of intermediate cells, but are still within normal range.  You have to put other factors into consideration, of course, such as chromatin pattern, nucelar membrane, etc.

You cannot use parabasal cells as reference because in young women, pap smars most time only brush off superfical and intermediate cells, no parabasal cells are seen.