I can see that I can config the width to either columns or px, could it be possible to set it to 100% instead?
Or can I target the message input box via a CSS override and fix it?

Hmm, not really sure what happened, I had tried the tip in the FAQ, but that didn't work, then I discovered that the value I had set in the config was still used, despite that I had removed it and saved, so I saved the uddeim config again, and now it works, very odd, I seem to see that quite a bit in the Uddeim config, I have to save twice to make it go through.