point of clarification
Robert Hutchins
>Howdy Joe,
You are not the first to struggle with something very elementary that SketchUp made arcane. DAMHIKT.
To set dimensions, immediately after releasing the mouse button when drawing a figure and before doing anything else, look in the window in the lower, right corner. You will see x,y coordinate lengths listed there. Use your numeric keypad to type in the values you want using a comma to separate them. Look first because IME which dimension comes first isn't obvious. I 'think' they always show the larger dimension first, but I've never read this anywhere - just guessing. You can do the same thing after using the push/pull tool to make your object 3D - type in the number after releasing the mouse button and before doing anything else.
You can also enter numbers as either decimal values, '4.25', or fractional values, '4 1/4' (note the space after the whole number, before the fractional part).
As others have said, go thru the tutorials. They are fun, easy, brief, and uncomplicated but they teach you the basics that you really need. One of the most useful things I learned was how to connect different parts of a drawing very precisely using the inference capabilities in SketchUp. Another useful feature is the ability to triple click the mouse to select an entire object. I use this a lot to select a component I want to move or connect to another component. You aren't likely to discover and connect this information to your actions without doing the tut's.
Please don't take this wrong, but don't be impatient and try to figure the software out intuitively (like I did). Many of the things are done very elegantly, but you have to pay the price of acquiring a bit of the plateau on the learning curve. Don't worry. It doesn't take long to use it.
HTH