Inside UI Dialog opens not to top but changes high of dialog

I am using selectric inside of UI Dialogs. If the select is near the bottom in the dialog, it does not open on top but changes the height of my dialog. 
I want to open my Dialog always with fix height and can not find a solution with selectric. It may be more a problem of UI Dialog, as that tool tries to adjust the height based on the content. I dont know.
Can you help?
Bernd
