I've never really been a good "big-lvl" creator but I think I know how to make a good one.
First of all, how large do you want it to be? I could make you a lvl 100x70 (without weapons and eyecandy, only the platforms/springs/tunnels) if you want to see how it's done.
If you don't want to see my lvl, here's a couple basic things you should do:
1. Look at other large lvl's like ForceDraw and Smog. They might not be very good one's but at least they show you how you can make a large lvl and still find ppl relatively easy.
2. As Dishguise said, put more tunnels and less platforms in your lvl. Make the open spaces masked but make sure you put a lot of eyecandy/stuff in there or else the lvl might look boring.
3. If you are gonna make lots of tunnels, don't make them too thight or the lvl's gonna turn kind of "one-way". Make them BIG and you could perhaps put smaller tunnels or platforms in them.
4. Suckertubes, warps, float ups and springs are all good examples of how you can get from one area to the other
fast. Put a warp on each of the four corners of the lvl so players can move from one side to the other. Float ups and springs are primarily to move the player up so if it's a high lvl, you might want to add some (or lots). Suckertubes are more interesting although it does ruin gameplay cause the player can't control him/herself. Make the suckertubes short and fast (but make the end of the sucker-tubes slow if you don't want ppl to crash into the roofs).
5. Oh, I almost forgot, look at Blade's lvls. They are HUGE (most of them

) but you can still find the other players (and in my case they can find me

) very easily.
6. When building large lvls, ALWAYS remember to mention my name.

(just kidding)
Alright, I guess this is all. Good luck on yer lvl and PM me if you want the lvl...
Cheers,
White Rabbit.
*I'm beginning to write replies as if they were reviews...*