You have to forget that ide'a about understand before doing at the moment, just do it and you see what happens, that will make it more easy to understand, and only what you need to understand in the beginning, sort of.
If i make a rectangle in 2D and select that rectangle afterwards, and then TAB to X Field down on Inspector Bar, type in a number in there that is different to what stand there already, and then hit the enter button, the rectangle will change size in X direction.
I dont need to understand how/ why that work, but i learn if i do so, it work, the rectangle change size in X direction, so then i know the basic of change size of any object, 2D or 3D, the principle is the same for all of them, only vary if it is in X or Y direction, and for 3D we also have the Z direction, but the principle is still the same.
And also, after you have selected a tool to do something, look down in left corner on your screen, TC will tel you what you should do next, or expect you to do next, some times not easy to understand, but most of the time you will get a clue about what TC try to tell you.
So 3D, a cube, same thing, first change view to an Isometric view, select the box tool, TC will tell you to click some place to set the first corner, after you have done that, TC will tell you to set the second corner, when that is done drag your mouse somewhere ,up and to right or left, down and left or rigth, thats up to you, after click the second time, press down the scrollwheel on the mouse if you want to tilt it more, and move the cursor slowly up or down and you will see the object change from 2D to 3D, when you have the higth you want, click again and the command it finish, you have drawn a Box.
Of course after you have set the first point you can TAb and write the numbers in the different Fields if you want to have it accurate
No repeat that a couple of times and make a few more next to each other, then select one of the box'es, press character D on you keyboard and your cursor change to a hand with a yellow point between it's "fingers" move the cursor down to one of the bottom corners on that Box so that the yellow mark is close to a corner, very close, keep the cursor there and press the character V and you will snap to the vertex point there, now you click on the yellow mark and move your cursor up to a top corner of one of the other box's you just have made, when the cursor is still over that top corner you press V again and you have just moved/ snap'd 1 box on top of a second Box, and the two corner is exactly on the same line in Z direction even if they are not same size, on that spesific corner they meet exatly on top of each other.
I would also turn of the Running Snap's by mark the mouse up on the top menu, that will turn it of and you dont have all the different places on an object where it is possible to snap to, only tha SEKE snap will work now, click on the mouse one more time and you turn it on again, it there is another snap symbol showing there and not a mouse, click and keep the pressure on it for a half sec ot so and a menu will open and let you click on the mouse.
Now click outside of the box'es you have just made, keep the mouse down and drag a window over them, and click in the opposite corner, now you have selcted them all, rigthclick and choose Property in the menu that just opens, then you click ob Box and can change Length, Width and Higth on all of them in 1 time and all of them will be the same size after you have done so and clicked the OK button