The key to precise sketching and modeling is dimensions. Any sketch or body you create will have a dimension label attached to it. You can select the sketch and then the dimension label, and adjust the value in the numpad.
These are the dimensions:
The length of a line.
You can also change the distance between two points by selecting them and entering the desired length (absolute, horizontal, or vertical).
You can also change the distance between a point and a line.
The radius of a circle or arc.
The angle of an arc or the angle between two connected curves.
To change the angle between two connected curves, select the two arcs or lines.