When you have the width filled, you can't define a height. This kinda makes sense, but I would like to be able to define both, and have them operate as "max-width" and "max-height". But that's only nice to have.
There is a bug in the current structure, because when you have width filled, and you want to switch to height, you can't. When you fill in the height, it gives an error that both can't be filled at the same time. But when you set 'width' back to 0 after filling 'height', it says it must be between 1 and XXX. If you leave it blank, it says it's not a valid property value.
This is still happening in 9.5.0 and a bit annoying to always add a new image if you want to switch. Have you raised an incident?