It will be annoying to get used to at first, but you'll end up having to move all your custom walls and objects using the 3-Axis mode instead of picking them up and placing them.

Hit Tab to switch to it, then T for moving, R for rotating and F for resizing (you can resize all 3 of the axes individually by clicking and dragging the colored boxes, or the entire object by dragging the white box in the middle).

While in 3-Axis mode, you are able to select the specific object you want to work with by clicking it. Its outline will be orange. You can select multiple objects by holding CTRL and clicking on them. You can then deselect them by CTRL-clicking on them a second time, or clicking in empty space to deselect them all. With multiple objects, any moving/rotating/resizing you do will apply to all the objects at once.

Hold CTRL while you are moving the object with the 3-Axis mode and it will snap to a grid. Each grid stop is 1/16th of the full size of a wall/floor. You can use this grid to better align your walls to floors, ceilings, etc.

Holding CTRL while rotating will snap the rotation to 15 degree increments. You can rotate the object 90 degrees by snapping it 6 times, 180 degrees with 12, etc.

To copy a wall/object you've already resized and you think is good to go, you can hold SHIFT while moving, rotating or resizing it in 3-Axis mode. The original object will stay and you will be working with a copy of it. You can also hold CTRL to snap it to the grid at the same time. This is a GREAT way to make sure all the walls/floors/etc stay exactly the same size.


SOME TIPS:

The square wall is 16x16 units, with 1 unit of depth. If you are making a replica in blender (or whatever else), make sure the cuboid adheres to that. For the walls I've made, I go by 1m x 1m x 0.0625m, but 16m x 16m x 1m will also work. Obviously for the half walls it would be 16 units tall and 8 wide, or 8 tall and 16 wide, etc. It should also be obvious, but floors/ceilings are literately just walls rotated 90 degrees, so the same units apply.

Once you import a wall, I would recommend resizing it to the exact dimensions of an in-game wall and placing it to the side of the editor so you can duplicate it (by selecting it in 3-Axis mode and holding SHIFT when you move it) when you need a new one. Make sure it is still inside the editor floor, otherwise the game will remove it when you save the map. I do this for objects as well. Resize them so they "look correct" and then place them to the side for when I need a copy of them in the level proper.

To make matching the in-game wall size easier, I recommend placing your imported wall directly on top of an in-game wall using the grid snap, then setting your Look Sensitivity to 1 while in the 3-Axis resize mode. You can then slowly, but surely resize the entire wall (using the white box in the middle) until it's JUST ABOUT the same size as the in-game wall. If they aren't z-fighting each other, but still generally look the same size, you're good.

Most importantly, don't forget about Ctrl+Z to undo stuff you mess up. You WILL accidentally grab a wall instead of moving it in 3-Axis mode, and undoing that is the most important feature.