Depending on the architecture, the immediate for branches may mean a byte address (x86) or a word address if fixed width instructions are mandated, which means that for a 16bits offset, the range is something like -2^17 to +2^17-1
Having fixed 64bits-wide instructions is quite rare (VLIW domain). If you add 32 bits, and if you keep the same number of registers (hence 5 bits for RS and RT) and the opcode part, I guess these extra bits would end in the offset part, so 16+32bits offset.
If instructions are fixed width, you may then have doubleword offsets : -2^-50 to +2^50-1
Practically, it does not make any sense to have such wide immediate offsets. No program is large enough to be several gigabyte long (I'm not counting libraries, which are generally called through indirect or direct branches over the full 32bits or 64bits address range, not conditional branches).
I could imagine that with 64bits instructions, some variants of the conditional branch instructions would offer 16 to 32bits immediate values, like : "BNE R1,#1234, @Somewhere" instead of using 48bits for the offset.