Have you ever wondered how small a line you can draw in Shape? Well, I did and I found some interesting results.
You have a choice of using either a "metric" grid or a "US" grid for your models. In either case, the first step is to zoom in as far as you can. In both the metric and US grids, the resulting major boxes are divided into an additional 10 boxes per side. These are the smallest boxes you can see. The actual smallest line segment you can make will be different in metric and US models.
Starting with the metric model, even though each of the smallest box sides that I can see is .01 mm long, the smallest line that you can draw is .02 mm. You just can't get it down to the .01 mm size. Even if you draw a .02 by any length rectangle and then draw diagonals to get an intersection point at .01 mm, you still can't draw that line from the intersection to the center of the .02 mm side. Of course this is all just hooey anyway, since there aren't many times that you want to draw anything that small - .01mm = .000393" - well beyond tiny! You CAN draw a line that is .02 mm long or .00786".
In the US model, the smallest box side you can see is .00625" long and you can draw a line that long - with some patience. Even with snapping on, it is a bit tedious. So, small can be even tinier than in a metric model.
There is one big CAUTION that I must mention in US models! If you use the line division technique (draw a line of a particular length and then use the resulting center point to draw a perpendicular line, which then results in another center point, etc.,etc.) then you will run into a serious problem when you make the smallest division possible down to 1/128th". If you try to draw a perpendicular line using the 1/128" center point - IT WILL ACTUALLY NOT BE PERPENDICULAR! It just won't do it. 1/128" is .0078125" long anyway, so stick with the .00625" box side and you'll be OK.
I realized about half way through this exercise that I must be nuts. Does anybody really care how small a line you can make? Probably not, but I did learn some interesting things! 
Please feel free to prove me wrong with a technique that will make an even smaller line possible. Then I will know that there is someone else out there as crazy as I am!