What was the largest tribe at that time, the smallest tribe, and what tribe was not registered with the rest and why?

Answer From The Annual Reading Schedule - Week 38 Notes For Numbers: See the below table and chart created with data from Numbers 26:

The Lewites (Levites) were not registered among the other children of Yisra'el because there was no inheritance given to them. However, the number of males from a month old and above was given (Numbers 26:4-63).