None of my dimensions never come out the same size? (Units-Inches) I have certain dimensions and measurements that need to be applied to an object. The height is 79 inches. 

Whenever I add a cube and go S-Z-79-ENTER, the sizes are never correct. 

As you can see in the screenshots, I have one modeled cube that was entered as 79, then once I added another it's so much smaller.

What am I doing wrong?
You are scaling the cube, not changing its dimensions.

When you scale an object, its dimensions are multiplied by the number. So your dimensions are not 79 inches; they are 79 times their original size.

To set absolute dimensions, change the numbers under Dimensions.