Karim,
Check the site:
www.highvoltagefootball.org
Down the left column is a tab that says, "NHFL EQUIPMENT RULES."
On that page is this...........
PLAYING FIELDS - Vintage 620 size fields are permitted. These may be stock or custom built. Larger boards are optional and must have an inbounds area no longer than forty five (45) inches and no wider than twenty two and one half (22.5) inches wide. All boards should have "pro style" hash marks. All boards must be deemed playable either by competing coaches' agreement or a league representative.