For the record, I got a local person to repair it for me.
One gotcha: when changing values, one must delete the .sparsebundle file first. Otherwise it will reuse the one previously created, with the previous values, and new values will seem to have no effect.
3500000 seems to do it. Let's see. But notice how the input field is not very wide (4 chars), making it hard to enter such large numbers.
"3500 g" also failed. It says there's 118GB of 499GB.
3500GB did not do it either (it says 2.34MB)
3500G is interpreted as 3.67GB, it seems :-(
Max Size for 3.5TB TimeMachine?
Left bay not working