Solution:

Due to the large number of available display resolutions, Webex does not recommend a specific screen resolution for optimal viewing. If a document or presentation is shared using the Share File (Including Video)... option, the presentation or document will automatically be scaled for optimal viewing on the attendee's monitor.

If you will be sharing your desktop or application in the meeting, you can change your resolution prior to joining the meeting to provide a better viewing experience to your attendees. The most common resolution on new computers is 1366 x 768.