To draw a straight line of an exact length in the Designer:

1. Turn on the Grid & Snap (in the controls at the Designer's lower left)
2. Select the Line tool
3. Utilize the snapping to draw a perfectly horizontal or vertical line (you can rotate it later)
4. If the desired length isn't a multiple of the grid size, you can select the line and set its length using either the Width or Height field

To draw a line with an exact angle:

1. Follow the instructions above to draw a horizontal or vertical line
2. Select the line
3. Use the Angle field and rotation buttons