Good morning,
I recommend you to use the feature info widget in a controller widget and set an action on your map widget that opening the widget as a popup instead of the original popup when you click on a feature. Just think to unactive the popup in map widget.
By this you can set the size of your popup like you want.
I second what @JasonBOCQUET said. Use the Feature Information widget then you have control over the size and placement.