Use the Styles Editor to set the height / minheight / maxheight/ width / minwidth / maxwidth of a Panel.
Drag on a Panel, right click on it and select Styles Editor
In the freetext CSS region, found on the righthand side of the Styles Editor, add the following CSS:
height: 800px; width: 600px;
Preview the app. The panel is now rendered with a fixed height and width of 800px x 600px.