First thing I would do is change your units to points (pts) as these read in whole numbers. You can change the width of a layout in the Inspector as long as you don't have any part selected. Click anywhere in the grey area and you will see the width box awaken, so you can change it. I guess the logic is that the width applies to all parts of the layout. The converse applies to height—it will be greyed out unless you have a part selected, as height applies only to each part.
Clicking the interior vs. the part type did it.