Creating Walls... That should be simple. Click on Ribbon > Home > Walls and away you go.

Aha. So you might think.

Well, I've been having this weird problem that any wall I drew had a fixed bottom and top offset of 500mm, even when I changed the height in the drop-down menu before drawing the wall.

It took me a while to realize that I had previously created a wall by using the Create Similar command in the context menu and that the wall I used as a template did have a bottom and top offset of 500mm. So when I used a different wall as a template (without offsets), I got the "normal" behaviour back.

Good to know that Revit remembers these things, even after coming back the following day.