If you are adding a new window or enlarging an already existing window you need to understand a few things about the construction and design of your home before you start your project.
The first thing to think about is the structural integrity of the wall(s) that you are putting the window(s) in. Your home is designed to carry loads (snow and shear to name a couple). Every component in the construction of your home has a purpose to counter certain loads. When adding a window you need to consider how the loads will be transfered within the opening. For example, for snow loads this is usually handled by the header that goes above the window. The size of the opening will determine the size of the header. It is best to consult with an architect or engineer if you have questions.
The second thing you need to consider is the thermal envelope. This is what helps maintain your inside temperature, regardless of what the outside temperature is. Think of it as the barrier between the inside and outside. This barrier consists of the inside finish material, the framing materials, the insulation, and the outer framing materials. Each of these components has a value, which is used to calculate how many windows, doors, and skylights you can put in your house. It also tells the architect that designed your home what size furnace and air conditioner to install in your house. If you have too many windows, doors, etc. installed in your house for the amount of insulation you have installed that may make your furnace work overtime to keep your house at a comfortable temperature.
There are programs available to check to see if your house has the proper balance of components. However, for the do-it-yourselfer it may be difficult to know all of the information needed to enter into the program to make it work properly. A good rule of thumb is if you are in doubt, ask a professional.