For movement, you'll want to decide on what the 3D units represent. Meters is a typical one (But use whatever is familiar to you, and you can imagine). Ok so, let's say every unit is a meter. Then in the 2D world, using something as a reference, you want to decide on how many pixels represent a meter in that world. Create a sprite box that will represent a square meter. Then adjust it's size accordingly.
Say it's size ends up being 130x130 pixels, and you're happy that it would cover a meter in your 2D world. You now set a variable or constant to hang on to the pixels per meter. Converting everything then out of the 3D space you would multiply it by this ppm (pixels per meter).
If a unit rolls along in 3D space 2 units in some direction, and you're using it's position to locate a sprite on the 2D canvas, it is 2 (3D meters) x 130 (ppm) = 260 pixels. That's the distance it will move. From there you just apply that simple scaling to all positions/distances. To take a position from the 2D world into the 3D one, you would just divide the pixels by that 130ppm.
As for boundaries, you'll have to determine what kind of projection best suits your situation, and using the pixels per meter method test out the boundaries. It's helpful to put around a bunch of debugging objects and see where they visually end up.
