According to 'Disclipline Established by Major General Howe for Light Infantry in Battalion, Sarum, Septr 1774'

Battn form at order The files to be at two feet Interval
........... at open order ...................... at four feet Interval
............at extended order .......................at ten feet Interval

And Ralph has written something more:
http://www.napoleon-series.org/cgi-bin/forum/archive2013_config.pl?md=read;id=147423

On Cooper, A Practical Guide for the Light Infantry Officer, 1806, p. 1-2:

LOOSE FILES.
When acting by themselves, and not in line, they may loosen their files six inches apart from each other.

OPEN ORDER.
Open Order is two feet between each file.

EXTENDED ORDER
Is two paces distance between each file, of any number of paces regulated at the discretion of the Commanding Officer. The files may be extended from the right, left, or centre, according to circumstances : in executing this, each front rank man must carefully take his distance from the man next to him, on that side from which the extension is to be made ; the rear rank men conform to the movement of their file leaders.