I'm not a designer but I do occasionally whip up my own images if the task is simple enough. One thing I've always wanted in a drawing program is the ability to create shapes by typing in dimensions. e.g. Dragging out a a rectangle to be exactly NxM units is so tedious and it comes up all the time. Does Inkscape or any other drawing program support anything like that?
Yes, you can have full control over SVG element dimensions in InkScape through input boxes as well as dragging by mouse. You can also use a snapping grid to define all your elements in relation to the grid, or enable many other types of snapping and automatic aligning.
Sure, Inkscape does that. It's a vector library, but if you create a rectangle, the coordinates and dimensions are available for editing. Type in "N" and "M" and hit enter, and it updates, which makes sense, given that those values are linked to the underlying SVG.
Yes. The shapes are xml and you can control them precisely. You can even control them with javascript should you want to embed them in a web page and, say, animate them or scale them.