To ensure that the width of the dropdown matches the width of the combo-box, you will need to use a DataTemplate for the items in the dropdown. The DataTemplate can be a TextBox whose width is set to the desired with.
The numbers that worked for me are
1. MaxWidth of the telerik:RadCombobox = 345
in conjunction with
2. DataTemplate TextBox width = 300
This made the dropdown width the same as the combobox