Which element has the greater atomic radius?

Looking at the m for size (greater m = larger size) works in these cases since the elements are in the same group. The general trend for going down a periodic group is that size increases. However, going across a period (left to right), the atomic radius decreases. This is because the nucleus is stronger (more protons) that it can pull the electrons towards itself more closely. In this case, more m actually leads to a smaller radius. Generally speaking, you shouldn't really be looking at m-size relationships; just remember the periodic trends.