Would the Br ion be bigger than the Se ion?

No. The Se ion will be larger than the Br ion.

The more negative the anion's charge, the larger the radius (more electrons = stronger repulsion between the electrons; so, more distance between them). Se would become "Se2-" as an anion, and Br would become "Br1-". Because Se's "2-" anion charge is more negative than Br's "1-" anion charge, and because Se would have more relative electrons to protons than Br, there is more repulsion between electrons, and distance, within the Se ion. So, the Se ion has a larger radius.

Hope it helps.