lobot got a reaction from Igor in Bionic apt update "File has unexpected size"
I've been having this problem off and on for a while now as well. The only solution that I found is simply to wait longer until more mirrors are updated and hope that one of the updated ones is selected.
@IgorHave you tried this: https://apt-mirror-updater.readthedocs.io/en/latest/readme.html
I realize it's still in beta but it may be worth testing.