 <p>The size itself does't matter, the whole point of a layout is that it should be reactive to the size available. Some items will be exact sizes and others will be proportional. The proportions are based around the overall size available and the parts that you need to be exact or to fit exactly to their contents.</p> <p>It's opinion but I guess the size shown in storyboard is big enough to allow easy layout but not so big it's hard to fit on screen with other panes for code and inspectors.</p>