The Inline Insert/Edit Form can be resized by the end-user for improved usability. You can enforce a minimum size by using theMinimumInlineFormHeight and MinimumInlineFormWidth properties of RadScheduler. For example:
<telerik:RadScheduler RenderMode="Lightweight" ... MinimumInlineFormWidth="200" MinimumInlineFormHeight="200" />
Note that the values are always in pixels.