The manual (page 119, 29.1 Representational scales) says:
Battlefield ground scales are based on maximum effective massed-firing bow
ranges when the standard representational scale is used. Thus 4 squares
represent approximately 240 paces, and each man in a close order formation
occupies a frontage of approximately one pace.
So one square represents then 60 "paces". And then we face the problem of what one pace equals in metres.
What I've found is the following: "pace = A traditional unit of length equal to 1.5 metres", which is consistent with the Roman pace (passus).
=> one square represents 90 metres roughly.