Thanks Daniel. Just played around with it and it works, sort of..
In the hotspots config under the Advanced Settings tab, I set max width to 290 and left height blank. This resulted in no thumb being displayed.
Then I set max width to 290 and max height to 290 - this resulted in a thumb 290px x 290px.
The height of my thumb should only be 140px. So I set both max values to 290W x 140H and it displayed correctly. I take this to mean that all of my thumbs in the info bubble will be 290x140 instead of keeping their proportions.