The window that opens with login screen is the widget designer that tries to authenticate the user in order do be displayed. The size of the designer is based on the designer template(for news is bigger than for lists) and the frame that wraps the designer is controlled by the backend theme(the pop-up is RadWindow). You can test with firebug to add
div.RadWindow_Sitefinity > table.rwTable
width and height to this css class. You can use the updated class with !important in a Css widget or by using a modified backend theme which is available with Sitefinity SDK.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items