How to Install sub-panel off main electrical panel.

I have several home wiring books and Ugly's pocket manual. The books give good advice on how to make the physical connections - no problem. What I can't understand are the values for wire sizes and circuits breakers. Ugly's gives lots of formulas but I can't understand how to plug in the values. House has 240V current on meter, 150 amp breaker on main panel. Subpanel off main panel will control three bedrooms and den. Questions: 1) what cable size needed to connect subpanel to main panel? 2) What size (amps) main breaker on subpanel? I can approximate total watts on the many outlets in the four rooms. Where does resistance come in, and voltage?
Thanks for any help.
Tom Graves

The answers to your questions are driven by the National Electric Code if you live in the US. You would best be served by asking at a local eclectrician supply business or on an electrician based forum. My local supply store has cheat sheets for DIYers.

Thanks Todd. I'll look for an electrician's forum. I thought the EE forum might not be the place for this kind of problem.
Tom Graves