On the last demo, I saw dashboard host page widgets of various sizes. I like the design of widgets in iOS where there are just three types of sizes: S, M, L.
Small is quarter size, medium is 2xS (wide rectangle) and large is 4xS (square). Apps can only specify which one to implement (most of them implement more than one size) and users decide which one to use.
All widgets are then flexible, depending on the parent content then can be wider or narrower, but the relative size is always correct. Hope it makes sense, I propose to consider the same and doing it rather earlier than later so plugin devs can decide the correct sizes now.