Now as you go across a certain period what happens to the size of the atoms: e.g. from Na --> Mg -->Al?

Answer 1

as you move left to right across a period, the atomic size decreases because the number of particles increases but the number of electron shell remains the same...so electrons are pulled more towards the nucleus (as nucleus becomes more positively charged) resulting in a decreased size.

Answer 2

GOING across a row atomic size decreases
and going down a period it increases

Answer 3

An atom’s size decreases from left to right in a given period.

for more information, see:

http://www.sciencebyjones.com/periodic_table1.htm

Answer 4

Decreases.