Having been in several gigantic minis games before (Apocalypse 40k, Warhammer, and big 40k games before Apocalypse), I have some tips. Not all of this is necessary, and some (or all) may cost money, but it can be nice.
Do you have a back yard or porch? If so,play there with a big table. This way, nobody (and no legos) are on the ground, and walking around is easier. Depending on the amount of stuff present, you may want to use a 4' x 4', 6' x 4' table or larger (we've done both 12' x 4' and 16' x 8'). To do this, Home Depot and the like sell large sheets of plywood, masonite, or particle board for 10-20 bucks tops. Considering the mileage one of these can get, it may be worth it.
Don't play on a windy day. Seriously, having paper and minis fly around can suck.
Plan on starting early and finishing late. Even in Brikwars, being outside, joking, playing can take a while to complete. I'd guess Brikwars with tons of units on either side would take at least 4 hours to play, if not longer. Also consider daylight, and having lamps if you will be playing late.
A tent may be a good idea, like a screen one or a super-bigass camping/event tent. Why? Shade is good, and keeping bugs away is better. Failing that, those off! candles or spray stuff may work.
Make food arrangements ahead of time. Either be super-generous and buy and cook all the food and drinks... or have each player chip in (someone brigns the buns, someone brings chips, someone brings beer if you're all old enough, etc). Plan for a lunch break (depending on the time), and take 30 at halftime for a little TV and eating or whatever.
In terms of keeping the interest for several hours, make the scenario you play fun and interesting. Perhaps there are controlable turrets across the map, and the ultimate objective is to activate the big mech in the middle. Or something.
In summary, big games are usually a big deal and investment of time and energy, but can be very fun if planned out a little. And can get repeat players and make a fun event to look forward to.