It'd to have a button in the workbenches (local, online) to allow us to apply the "layout" (different boxes sizes, flowing behavior, etc) of a team site or a communication site so we can double check rendering of our webparts work well with production dimensions, flowing behavior etc...

Today the only way to try to mimic that is the tablet mode in the workbench, the browser debug tools or just resizing the window. This offers a poor experience and doesn't guaranty me my webpart is in the same rendering context.