Set width and height ne vw,vh

There are units in CSS relative to viewport size: vw and vh.
But in vaadin 7.5.9 using this units in setWidth\Height causes
java.lang.IllegalArgumentException: Invalid size argument: “100vw” (should match ^(-?\d*(?:.\d+)?)(%|px|em|rem|ex|in|cm|mm|pt|pc)?$)

Is it by design or some bug?

Looks like an unintended limitation or, in your words, a bug.

Thanks for reporting!