Setting the RadPopupEditor.DropDownMinSize property is expected to specify the minimum dimensions that the popup is expected to have.
this.radPopupEditor1.DropDownMinSize = new Size(200, 200);
Hence, you can enlarge the popup, but it wouldn't be possible to shrink it to a smaller size. Please have a look at the attached gif file for better illustration.
I have provided my sample project as well for your reference. Please give it a try and see how it works on your end.
I hope this information helps. If you need any further assistance please don't hesitate to contact me.
Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik
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