I tried doing that offline on my windows, it also didn't give me the option to increase the size. which is strange, is there any configuration of limiting partition size ? you write to wrong place - that's a RockChip Rk3399 Linux, not a A64(I believe, it's a AllWinner - and thats a right place) ;) ...

The Ubuntu MATE OS you ended up at is probably not for the M1 plus, but just the M1. It installs but without radio functionality. I just went through this myself. Everything lists MATE as an M1 plus OS but its an error in the specs. If its not too late for you the debian wifi can be established by ...