Saga of Jaraah
The Player’s Handbook says a spyglass has only 2-3x magnification. Such a scope would be extremely rudimentary. Galileo, one of the pioneers in the design and construction of telescopes, achieved as much as 30x magnification using the crudest of tools.
For this campaign, a “typical” 1000 gp spyglass has a magnification of 20x and an objective lens of 1 inch. It is 4 feet long, but telescopes to a cylinder 1 ft long and 4 inches in diameter. The field of view is 15 arc-minutes (these are all specs achieved by Galileo. This means (roughly) a 1 foot object at 100 yards will fill the image space of the spyglass. This ratio scales linearly. At a mile (1760 yards), a 17.6 foot object will fill the image space of the spyglass. At ten miles, a 176 foot object will fill the image space and the viewer will have corresponding difficulty discerning small details.
For game purposes, a spyglass can allow its user a good view of a man-sized object at 1/2 mile. The viewer will not be able to identify the person, but will be able to distinguish hair color (if hair is exposed) as well as general armor (or clothing) type and the presence (but not necessarily the exact kind) of any exposed weapons size M or larger.
Jaraah is Earth sized, so typical calculations for distance to the horizon can be used. Assuming flat terrain, 1 meter means 2 miles to the horizon, 2 meters means 3 miles to the horizon, 20 meters means 10 miles to the horizon.