6x4 works as long as you have decent terrain. Most of the time at our LGS we use an 8x4 board. Usually the board is setup in a way that there are pockets of 'open' areas and pockets of heavy obstruction. When you have a board that has a coherent theme it makes it alot easier. Albeit I do agree with the issues people point out about 25mm games -v- smaller. For a Tournament (most tournaments) are 1500 points 6x4 board, 25% terrain. At that point cost its balanced. If its a friendly game, then just discuss whatever will make the most sense.
We've done 8x4 Spear-head, 8x4 longs but 4' deployments (each half). This allows Lemans to fire the entire length, but also allows close combat to start towards the middle of the board. Also if we find the game is getting too bunched up we just allow each other to do things like outflank with units that normally can't outflank. It's up to you and your opponent to agree to what to use. On those larger battles discuss it ahead of time. Often we'll discuss next week's battle the week prior then spend the entire week designing the army. Allow flyers to 'redeploy' similar to how a Mawloc redeploys (Return to reserve, deepstrike next turn without rolling for them to come on board) etc.
The game is based around 1500 points, 1 FoC, 1 6x4. What everyone is saying is they want MORE. If you want more, then just do more. The last game we ran, was a city seige. Entire board was coated with terrain. Buildings, roads, debris, etc. The only open areas where peices of road not cluttered with Terrain. The game was a blast and exceptionally tactical. It was hard for both melee AND shooting. There was no safe deep-strike zone, there was lots of areas to hide, and there was alot that hindered movement. It was themed.
I understand many people want to play exactly word for word how the rules are written and often play against people they don't know. For those games, just use the standard 1000 - 1500 point games on a 4x4 or 6x4 board. Once you've gotten to know the people through social interaction (A huge part of war-gaming that we often overlook) then talk about different battles.
For example, I'm trying to design a Tyranid -v- Space Marine battle similar to Starship Troopers last stand at a bunker. We're going to allow rules that if a Marine isn't supporting melee fighting at the gates (More than 2" away from the people engaged) then you can split fire at enemies not in melee combat that you can see (Such as large bugs). The marines will deploy (no vehicles) in a bunker about 12" x 12" in the center of the table. The tyranids will assault from all sides. We're not done designing it, but that's an example of creating a themed scenario utilizing 40k rules while also supporting things we think would make sense in that final battle. WOuld we run that for a tournament? no.
If the game ends and both sides are happy over a fun game, then you've both won. Everyone wants to win but winning shouldn't come at the expense of having fun.