The following assumes a red or black letter on a white background.  Choice of color combinations and typestyles can/will affect legibility and visibility.

3" recommended for maximum impact @ 30', while maximum visibility is 100' 

6" recommended for maximum impact @ 60', while maximum visibility is 200'

8" recommended for maximum impact @ 350', while maximum visibility is 350'

10" recommended for maximum impact @ 100', while maximum visibility is 450'

12" recommended for maximum impact @ 120', while maximum visibility is 500'

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