Answer:
Al
Step-by-step explanation:
Seeing that both elements are in the same column (a "downwards row"), the radius will increase as you go down. Al is lower than B, so it has more protons, electrons, neutrons (hopefully), ect. This also means that it is larger. Note that this only works when going down. Going right actually usually (but not always) has the trend of a decreasing atomic radius despite the number of protons increasing.