Region3s are built with 2 points. Think of it as a 3d prism, and there are 2 corners directly opposite of each other which you can move around to change the size of the Region3.
You could use some complex math functions, or you could use a module that does it for you. RotatedRegion3 allows you to construct a Region3 with a CFrame and a Size because this module allows rotated region3s. Roblox Region3s are square, and therefore cannot be rotated, meaning that it only needs two points to create the region.