A good ruler or stencil is:
- in the correct shape (!)
- has a smooth edge with no flashing or nicks (plastic edges occasionally have a bit of flashing from the molding process -- they can be carefully cut off with a knife)
- easy to align with preexisting drawings
- easy to keep flush with the writing surface, to prevent slippage
In my experience, the size of the ruler or stencil is not a major factor, though because I often draw in a spiral-bound drawing pad, smaller edges tend to work better. Larger edges are heavier and therefore better for prevention of slippage.
Of course, if you use a drafting table and t-bar, many of these issues (at least those pertaining to straight lines) go by the wayside.