Hmmm... without spoiling you too much..
You can try to map out the maze, I know I attempted it once on graph paper and I got hopelessly confused
But, then again, I'm not the best with navigation...
You can also make notations on your in-game map, so what I generally do is use a number system essentially based on the order in which I enter each room. That way you can more easily keep track on where you have been.

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OK.. though it may be difficult, I very strongly suggest you keep it on the hardest difficulty, not just for the experience either (see spoiler below if you want to know more). I also suggest you try to travel in a generally northerly direction.... Any treasure you find is purely icing on the cake and not the real point at all..
Note that each time you reset the cube, everything you have done will be erased. Also handy is that you can actually use the cube as a convenient resting spot, and once you find a transporter stone (you'll pick them up as loot throughout the cube), you can also employ the cube as a travel device.
There are two places you need to find, highlight below if you want to be spoiled:
You want to find a joinable NPC called Nordum.. and you also want to find the director's room, hopefully in that order. Nordum is a modron cube himself, sort of, and one of the best NPCs in the game
End Spoiler.
2. Can I leave and come back. So I can sell off items? I think you actually have to talk to one of the modrons to reset the dungeon, but I still wouldn't advise leaving any items in there.
3. Is the normal area bigger the the easy and the hard bigger then the normal, or are the monsters just harder? Have only been in the easy dungeons so far.
It doesn't matter what setting it is on, the areas are all identical, the difference lies in... what you can find.
I hope that helps

It's a while since I've played the game so some of my details might be a bit vague, but it will steer you in the general direction anyway.