Now that you've decided to invest in a new piece of furniture, let's make sure it fits perfectly in your room.
Follow a general rule of allowing 36" of traffic pattern around an arrangement of furniture for traffic flow.
If your cabinet has to be turned on its side to be carried through a standard door into your room, make sure it will clear the ceiling when it's placed upright. To calculate the diagonal measurement of the cabinet, dust off the Pythagorean Theorem: height of the cabinet squared + width of the cabinet squared = diagonal measurement of the cabinet squared. If the square root of the diagonal measurement is less that your ceiling height, then you're good. If it is more, then the cabinet will not fit into your room unless it is made in two pieces.
For coffee tables, rules are made to be broken, but these three rules should give you some structure to work with: generally speaking, you'll want your coffee table to be roughly the height of your sofa deck (about 20"), roughly 18" from the sofa, and approx. 2/3 the width of the sofa.