I get the same error when using the up-and-down buttons to change the size. When I enter a number (which is normally the way I would do it) I can enter it just fine but nothing happens.
That's helpful info.
I tested only the arrows ---> error message.
Typing in numbers functions just fine for me. I can type them in and the size changes accordingly.
It does confuse me though why nothing happens for you? Maybe it does function but you just haven't noticed the size change on an empty black level.
Otherwise typing in the numbers could be a workaround until the critical problem with the arrows is fixed.
Edit: Ok, after changing sizes several times now while investigating the other bug, typing is 100% a workaround.