Not having similar screen resolutions (screen size settings) may result in your attendees not seeing all of your shared application/web browser, or it could make the screen too small to view comfortably. Usually it can be corrected by everyone setting their screen resolution to 1024 x 768.
This setting can be changed in the Settings section in your "Display Properties".
Right click on and empty area of your desktop and select Properties from the short cut menu to change this setting and click apply.
Attendees can change the view inside the meeting by clicking on the drop down arrow on the right side of the drop down bar and selecting View. From this you can choose how the application or web browser fills your screen.
Click here to see an example of the Display Properties panel in Windows XP as well as Windows 7 and how the attendees can change their own view.