H- is larger than Br- , bcz in case of H- no.of electron=2, while no. Of proton =1 ;so the ratio become 2. It makes H- highly polarizable (and due to this reason,it acts as a very good bridging ligand in organometallic clusters)… now, polarizability directly proportional to volume ; greater the polarizability, greater will be volume, so larger will be the radius.Now, both theoretically & experimentally, polarizability of hydride ion is greater than bromide. Therefore, H- is larger than Br- in size.