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There were no max length on halberds. I believe the one we used in 9th Stirland were around 2.6m. incl. head. We intended to make them a bit shorter, but a mistake in communications meant they got 10-20 cm shorter than planned. Having the longest halberd is a distinct advantage against other halberdiers, but against shields its not much of an advantage. I doubt they are effective if they are longer than 3m, unless you develop very special fighting tactics.

In regards to restrictions, they were mostly "the usual" from danish LARPing. That is thrusting and charging is legal (though people is obligated to use commen sense when charging). Setting weapons is not allowed, though there are no rule directly forbidding it - it is more of a commen concensus amoung danish LARP'ers.
In addition, we had a "new" rule in regards to the amount of physical contact. Usually, shieldbash, bull rush, etc. is a topic of dispute, as some people think it is okay, while some people disagree. At the event we had a rule called "wood against wood". It basically stated, that it was legal to strike wooden parts against each other, but forbidden to strike wooden parts against flesh/persons. So shieldbash against other people were allowed, if they were armed with shields, and halberds could be used to block shields. I think that worked out OK.

Halberd construction? I'll see if I can explain.
For the staff, a wooden pole was used. Based on experience from this and other halberds, I'd say that they should be at around 1½ inch thick. We drilled a hole in the end for application of the head (explained later on).
The heads were made like tradtional weapons, with a core of glass fibre at Ø 9mm. The final 20 cm (or so I think) the glass fiber was replaced with a heavy rubber hose, used for pressure pumps etc. This made it stable, yet able to bend. The hook was reinforced with thin leather, so it wouldn't break (it is used to grapple shields and pull them away, so you can hit people in the chest).
The glass fibre core was a bit longer than the head, and was glued in the hole at the end of the staff. The upper 30 cm of the staff was applied some foam, which in turn (for looks) was wrapped in either green og yellow fabric. This is to avoid striking people with the staff.
This halberd is usually termed "stagevåben" (in english roughly translated to "polearm"), and is mainly used for stabbing and thrusting. For cutting/chopping (as you would with a real halberd), the padding down the staff should be more than 30 cm - rather 100 cms or the entire length of the staff.
That was the ultra-short guide to polearm making.