Yes, the block size can vary even on a steady state. In particular, once the state proofs feature would be complete, some block headers ( usually at a constant interval ), would contain the compact certificate, which could be pretty large.
Unfortunately, I don’t have any concrete numbers to provide. If you want to find the block size, you can use the goal utility to dump a block in msgp format, try to remove all the non-header data, and then try to rebuild the msgp data.
Note that while the node does (currently) store the block information in msgp format, it’s not guaranteed to remain the case.