I created a rectangle, and I can change its dimensions:

[Oh my god, how to insert image here?...]

https://i.gyazo.com/4846e52744dee63ce13e9b71aab07bab.png

Now if I rotate the figure:

https://i.gyazo.com/3655581a1a9b1bec63849863ff7769a3.png

I cannot change width or height of the object. 

Is there still a way to do it?

Hi.

Yes. If you select the rectangle tool while that rectangle is selecten and when it's still not converted to a path you can still type in the height/width values in the spinboxes at the top.

Lazur

Yes. If you select the rectangle tool while that rectangle is selecten and when it's still not converted to a path you can still type in the height/width values in the spinboxes at the top.

Ah, thanks, it makes sense. However I would like to have the ability to change dimensions in the coordinate system bound to the object.

Inkscape does not have the ability to change the coordinate system relative to a specific object or area of the canvas.

Although I think in the upcoming version 1.0 it will be possible to rotate the whole canvas.  I would assume the coordinate system would rotate right along with the canvas.  So possibly there would be some way to use that new feature for this.  I have not experimented with it myself.

There is a way to edit a rectangular path (or shape) after it's rotated.  But there's not a way to do it by specific dimensions -- only by visual estimation -- or snapping.  If you could set it up (or it already is set up) it might be possible to use snapping to do it.  I would need to see the specific SVG file, to know for sure.

Actually, that option that Lazur described, using the rectangle tool, does use the actual rectangle's width / height - it changes the length of the rectangles sides (as in side a and side c = width, side b and side d = length). These values can be entered into the number fields of the rectangle tool. You can also edit those on canvas, by using the rectangle tool with an existing rectangle.