Inkscape: How does one modify the current canvas or page with custom settings rather than the list of predetermined ones that is offered; alternatively, how does one access the equivalent of a crop tool?

Ok. I wish to make a drawing whose objects fit within a page whose proportions I determine. There is a possibility of changing the page, but I can see no way of inputting custom values.

To state it otherwise: I wish my drawing to have proportions which I can parametrize and in which the object elements lie where I wish them to lie.

How do I do that?

Also, and related, how do I access the equivalent of a "cropping tool" which allows me to crop with the freedomof choosing my own crop proportions?

Unrelated: there is an excellent 3D box tool. Is there the equivalent tool for 3d objects which are derived from a circle or from other polygons, not just from a square?

Thank you!