Drawing dimensioned objects

Dimensioned Objects
Setting the Current Drawing Preferences to Paper format/Letter
I would like to draw a rectangle (say 30" x 50") by:
1) Drawing a rectangle and then giving it width and height dimensions.
or
2) Drawing a rectangle w two points, viewing width and height dimensions change as I move the cursor, and using that information to determine where to click the second point. No matter what Drawing Preferences/General Scale settings I use, none of them seem to make a difference as far as the units speeding by in the bottom left corner are concerned.
3) Placing a rectangle, selecting it, finding its listed dimensions, and changing them. (in other words, not defining the rectangle through command line, just drawing it, and changing the dimensions)

How do I do each way?

When I do a print preview, I notice that the rectangles I've drawn seem to automatically be positioned so as to take up a single Letter Size page. (which leaves me wondering how to set up and print a multi-page drawing)

Note: I can draw a tiny tiny rectangle on the grid and zoom into it, but it seems I should be able to adjust the grid so that Letter size sheet takes up approximately full screen and I can approximately position all the elements of the drawing and size them appropriately.

Thanks

You can't change "listed dimensions" of a rectangle afterwards. I know, in other CADs you can, but not in LC. For some entities you can edit some values via Properties, but not for a rectangle.
Determining the dimensions of a rectangle by view, watching the coordinates, is not a recommendable way. Too fuzzy. You might use Snap to Grid, better, but still you can't get any desired value.
Best determine the values by input to the command line while drawing.
Please look at the manual how to do it, not everyone can be given an individual lesson.
https://librecad.readthedocs.io/en/latest/index.html

You can't set the grid to a certain sheet size. It is recommendable to draw a rectangle in that size. If you intend a scaled drawing then draw the rectangle accordingly x-times larger. Then do an Auto Zoom, and you will have your sheet approximately filling the screen.