Can't confirm if this is mentioned anywhere so I wanted to let you know just in case...
For a RadDropDownList that isn't currently visible (it's added dynamically to a panel which isn't currently in focus even though the parent form itself is loaded and visible) the .Bottom and .Height properties don't return the correct values if AutoSize = true. .Height =0 and instead of giving the correct .Bottom value it returns the .Top value - I assume this is because .Height = 0.
If I leave all code exactly as it is but set AutoSize = false then .Bottom and .Height are calculated perfectly.
This is pretty counter intuitive since the RadDropDownList is 100% established except it hasn't yet been displayed. At this point there are no items in the list though so I'm guessing that might be breaking the AutoSize.