Separating out /boot will be problematic if you ever want to restore or boot off of snapshots. You will end up in a situation where the kernel version is different than then the kernel modules.
If you want to do it anyway, the size will be based on the number of kernels you want to have installed. You should probably budget a little over 100MB for each kernel/initramfs.
It sounds like you may be referring to the EFI partition which is different than /boot on a legacy install.