My 2 cents:
Multi day would be a very small crowd, as jake said if I can scrounge up 1 day to play every 2 weeks it is fairly lucky.
Depending on the crowd you want to attract, the size and format are important. For the local GT players and other fairly competative groups, the 2250-2500 range is good. Also using ETC restrictions is a great way to get more people as many players love to "practice" that format. Also it tones down the crazy stuff and buffs the weaker armies.
On the flip side, if you wanted to make it more assessible to newer players, teams are always fun. Like 1250 a team would make for decent games. Teams tend to naturally cause the games to be wacky which limits needed restrictions.
One thing I have thought of : what would people think of a Warhammer mixer of sorts?
This is what I mean. Every shows up with 2 1250 lists. Teams are randomly matched and between the team, they pick what lists to use for the whole event.
The 2 lists are to help the issue of "we both brough lvl 4's". Which screws folks at that point level.
I think this might help newer players feel more comfortable with a teammate, and get the community to meet each other.
There are some gaming groups in the area that might be interested in coming by for the tourney (from CT, Cape, etc).
Happy to spread the word when we know the format.