There is no requirement - only the race officer's judgement and (usually) the ability of the gate mark boat to set marks where the RO wants them.

In anything up to about 15 kts, I try to keep the gate at 7 BL (115 ft) for Hobie 16s. More wind = wider gate. A problem arises when you have multiple classes on the same course. 7 BL for Hobie 20s is 140 ft = 8+ Hobie 16 BLs. Spinnaker boats need an even wider gate, especially when the wind picks up.

Setting the gate is difficult when your average GPS is only accurate to ~15 ft - assuming your gate boat driver knows how to use one. Deep water is problematic, too. At the Soling Canadian Nationals, we were setting marks in 170 feet, making the gate set very tough. Current can play games with you too, as the marks will "skate" around in a circle if the current is opposite to the wind and the windage of the marks = the drag of the current. You have to minimize the scope on the anchors, while still keeping them in one place on the bottom.

As far as getting the gate square, Eric is right - the wider the gate, the more the competitors are likely to treat it as a single leeward mark by favoring one side. Nirvanna for the RO is getting an even split at the gate. It's a lot harder to do with monohulls than it is for catamarans. I will adjust the gate on every lap, trying to get the even split.