There is no maximum height for your wall, but keep in mind that you want parts of walls to disappear when you look at lower levels, so make walls per-level and set their level flags accordingly.
I usually stick to the following convention:
- Make floors either 8 or 4 units high (one or two squares on the 3 grid). For normal floors stick to 4, make it 8 when you want to make a sturdy-looking bridge or something.
- Floors should always have their base at the bottom of that level (0 for level 1, 64 for level 2, 128 for level 3, etc). On your niveau04 map, the bottom floor is actually below 0. Try to avoid that.
- The bases of walls should be raised to sit on top of the floor. Alternatively, you can make the walls reach all the way to the bottom of the level and make the floor brushes go around the walls. That looks better if you don't like the floor brush showing at the sides of the map, but you need more brushes for the floor.
- No ceiling brushes.
Hope that helps.
