Several factors play a role in your choice of letter size but the most important is readability.
Use the following chart for realistic distances at which letters can be read. Note: logos are an exception to the rule of type, if you have marketed well a logo will be recognizable before a viewer is close enough to actually read the copy.
| Letter height | Distance readable |
|---|---|
| 4" | 100 ft |
| 10" | 250 ft |
| 16" | 360 ft (city block) |
| 22" | 500 ft |
| 33" | 750 ft |
| 43" | 1000 ft |
| 57" | 1320 ft (1/4 mile) |