If you set the advert size correctly in the Group settings, that’s your override. If the theme still overrides that, then adverts will show up wrongly. Some themes use dumb things like !important for image sizes.
Believe what you want, but unfortunately in 90% of all cases the theme is usually doing this if set up the group correctly in AdRotate (Pro).
You could of-course easily check that for yourself by going through the browsers code inspector to see where the 100% width (and possible overflow) comes from and then see which CSS causes that 🙂