You can change the interface (window size) of Omnisphere by clicking on the “Utility” button, and in the following menu (see image) you go to “Magnify Window” and then choose a zoom setting.

Since I personally use a very high resolution screen, I usually have it set on 1.7x zoom. But some people prefer having it smaller so that it is not in the way. What will you choose? =)

Omnisphere Tips - Resize Window (Zoom)