How long is a unit of distance?

A rule of thumb is that Teemo is roughly 100 units wide. So, for your example, you could mentally translate the Buster shot ability to:

BUSTER SHOT: Tristana fires a massive cannonball at an enemy unit. This deals 300 / 400 / 500 (+150% of ability power) magic damage and knocks surrounding units back 6 / 8 / 10 Teemos in distance.

Basic attack range of Ashe will give you the best idea. Ashe can hit units from 600 distance away. In the picture below, the blue line is 600 distance long. That is the range of buster shot's knockback at level 1. At level 2 it is one-third longer and at max level it is two-thirds longer.

(Image taken at 1920x1080 zoomed out as much as possible)

You can also hover your mouse over the Attack Damage amount from the statistics right at the bottom, the game will draw a circle around your character which determines the auto-attack range your champion has.