Sight Range

PHB has Sight Ranges on page 155/156. We'll modify these by personal range, I think and make size of TARGET adjust them too.

Moving Target 1500 yards (can only see movement, not what it is doing)

Stationary Target 1000 yards (can determine that an object is there but no specific details unless unusually shaped)

Identify Target Race or Types 500 yards (general actions can be ascertained, size, shape, colour and creature type distinguishable)

Identify Individual Target 100 yards (individuals can be identified, providing they are not concealed, actions can be read)

Identify Details 10 yards (emotions and specific, small actions can be identified)

Visibility Level modifies (but how?)

For every size category of target, double the ranges. Groups of characters are converted to the nearest size (thus, enough Medium characters are seen as though they were Large). For every size category the character has below Small, half the ranges.

How does the character's own size effect how far they can see though? Modified by Personal Range.

The visibility level the character is subject to decreases how well the character can be read. We'll use a table for this!

Vertical column = visibility level

Horizontal Row = range of given category (from shortest range to highest range)

Vision and Visibility (pg 72/73) in Wilderness Survival Guide. For every Size category below Medium, double these. For every size Category above Medium, double the distance it is visible from.

Seeing Light in the Dark

(for every 10% Visibility a Light Source has above the area immediately beyond its radius, increase the distance a light can be seen by x1. This starts at 0, so it only becomes noticeable at Light 90%. Lights use Moving Distance if they are flickering when ambient Light is above 80% but otherwise use stationary distance. Below 80% Ambient Light, they always use Moving Distance. So, at 50% (moonlight), a light can be seen up to 1500 x5 yards away, or 7500 yards away. Thats about 4 miles, which means usually it'd be hidden by the curvature of the earth.

Make a table for this, too.

Curvature of the World

(on an earth sized planet, 3 miles is the limit of sight due to the curvature of the earth and our relative size. Alter this by Personal Range. Note that a DM is especially free to ignore this consideration, especially if they don't know how big their world is. On flat worlds, this is indefinite.)