When activating a size in Flexitive, the size will be activated by taking the layout of the active size that is the closest match, and adapting it's layout to fit with the new size.
While Flexitive will usually do a great job of choosing which layout to use to activate your new design size, you may not always agree with the decision Flexitive has made. In these situations, you can override Flexitive's defaults using the 'Inherit From' dropdown in the All Sizes View.
The Inherit From dropdown, you'll be able to choose any of the sizes you've activated so far in Flexitive. After choosing a size, Flexitive will take the layout from that design and force the size you're activating to adapt that layout to their size.