Scale

Distance increments on the map (tiles) must be small enough that the slowest unit (over the slowest terrain) can move one unit in one turn.

Turn length must be short enough to give players sufficient control, but not so small as to make the game repetitive.

Aimed rounds per minute

A possible determinant of turn length could be the time taken for an average (of fastest) unit to shoot one aimed round.

Web searches have suggested that an archer could fire 12 to 15 aimed shots per minute, and that a WWI rifleman could fire 8 to 12 (and exceptionally 15) aimed rounds per minute. Other sites suggest that 20, 30 or 50 aimed rounds per minute is possible.

The maximum of these would suggest a turn length of 1 second. For automatic weapons, one burst is fired per turn, with the number of shots determined by the rate of fire. So, for example, an AK47 can fire 600 rounds per minute, therefore 10 rounds per turn.