That really depends on how tall you are, what you're standing on, and how lumpy and cluttered the ground around you is. Let's assume, for the sake of argument, that you're levitating up over the vast and flat Billiard-Table Plains:
| Eye Level (in metres) | Horizon Distance (in kilometres) | Notes |
|---|---|---|
| 0.05 | 0.7 | Lying on your face in the dirt |
| 0.125 | 1.1 | |
| 0.25 | 1.6 | Crawling on your belly |
| 0.5 | 2.2 | Crawling on your hands and knees |
| 1 | 3.2 | Sitting on the ground |
| 1.5 | 3.9 | Average human eye-level, standing |
| 2 | 4.5 | |
| 3 | 5.5 | From horse-back |
| 4 | 6.3 | |
| 8 | 8.9 | Average storm-giant |
| 16 | 12.6 | |
| 32 | 17.9 | From a tall tower |
| 64 | 25.3 | |
| 125 | 35.3 | From the top of the very tallest trees in the Great Forest |
| 250 | 50 | |
| 500 | 70.7 | From an average hill-top |
| 1000 | 100 | |
| 2000 | 141.4 | From a small mountain-top |
| 4000 | 200 | |
| 8000 | 283 | From the top of the highest mountain peaks |
| 16000 | 400 | |
| 32000 | 566.6 | Not very much air up here |
NOTE: To make life easier for myself, I've assumed a planetary diameter of exactly 5,000 kilometres. All horizon distances have been rounded to the nearest tenth of a kilometre (100 metres).
I've used data from this site to get the relevant figures.
Naturally, obstacles like walls, buildings, undulations in the ground, trees etc. are going to have a significant effect on what you can see, as will the height of whatever it is that you're looking at. For example, although an average human can only see about four kilometres before the horizon intrudes, they'd be able to see the top of a storm giant's head almost nine kilometres away... assuming, of course, that they could distinguish the teensy-tiny black speck as a storm-giant's head.