Welcome to the forum!
There are several ways to approach making objects the right size and aligning them to the proper locations. Generally, the Selection tool control bar is used for making things the right size, although there are other ways. This will tell you where the control bar is, and a lot of other parts of the Inkscape window:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Anatomy.html Note the W and H settings on the Selection control bar, and also X and Y. You can also position things this way, but there might be better ways.
It sounds like you just need to spend some time learning the basics. Here's all the info in the manual about this:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Position.htmlEspecially since you want to send this to a cutter, you will want to be sure
not to touch the Scale setting in File menu > Document Properties > Page tab. Even though you might notice it doesn't say 1.0, don't touch it. If you've already adjusted it, we can tell you how to get it back to the proper setting.
If you get stuck, let us know, and we can help
Or else you might want to try working through some tutorials. The Home tab of this website has a huge listing of tutorials. And of course there's always the internet for searching. Normally I suggest Help menu > Tutorials > Basic, Shapes, and Advanced tutorials for beginners. But I don't think those will give you the specific info you need.