The ASPxPopupControl dimensions cannot be set as a Percentage. The recommended way to achieve the specified dimensions correctly in all browsers is to use Pixels.
Documentation - Update the ASPxPopupControl / ASPxDockPanel topic - Add information about using Percentage for dimensions
Answers approved by DevExpress Support
We have implemented the functionality described in this ticket. It will be included in our next update(s).
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We have updated the ASPxPopupControl Class and ASPxDockPanel Class topics with the required information. A new version of the topics will be available with the next documentation update.