Question

Arrange in order of increasing atomic size

the Period 2 elements Be, Li, and Ne

Answer 1

Look up their atomic numbers (Google maybe?)
arrange in that order

Answer 2

Anonymous does it the easy way; however, you can think your way through it, too. In the same period (row), all of the electrons are going into the same outer orbit, therefore, the distance is generally the same from the nucleus BUT the nucleus is increasing in positivity due to increasing the number of protons. Therefore, the outer electrons are more strongly attracted inwardly and the radius of the atom decreases. So for the Li, Be, Ne series you would expect Ne to be the smallest and Li to be the largest. Hope that agrees with the google numbers when you look up those values.