At the moment the webcontent spot style has no options at all to change this so you have to edit the xml. This is annoying because you have to remember to make these changes everytime when you make a change in the tour and create a new build. Ideally in the webcontentspot style you should be able to set your own default size for the content. I assume this is an easy thing to implement so I would like to see this in a next version of panotour pro.
I think the best implementation would be if you could set the horizontal resolution of the content and have the vertical resolution calculated in ratio of the spot dimensions. i.e. in the spot style parameters you set the contentwidth to 1000 Then when you create a webcontent spot which is 280 x 170 then the calculated contentheight would be: contentheight = contentwidth / spotwidth * spotheight = 1000 / 280 * 170 = 607