When you are viewing the Yard Planner, you can select 'Show Tower Ranges' to display the range each tower has. Range is showed in red along with the tower as a block in the middle.
Every 20 extra range from a tower is same distance as a block. If a tower has 200 range, it means that it has a distance of 10 blocks.
You may also toggle if you want to see ranges or not. Uncheck/Check the ground range box for ground-based towers, while Uncheck/Check the Aerial range for air-based towers. You can also see their levels if you check the Show Levels Box.