Gutter systems come in a variety of sizes and materials, all have pros and cons which I will go over as briefly but effectively as possible. Residential gutter comes in three sizes: 4", 5" and 6". The 4" gutter was the primary size installed on new construction before 1990; it was made from raw-unpainted galvanized coil that has a flange or lip that is turned into the back of the gutter itself that acts as a flashing. The flange has to be installed so that it rests on top of the fascia board and under the shingle to insure that water will enter the gutter and not get behind it causing a “leak.” The reason it was used for new construction was twofold, new construction is relatively level and a level gutter will "drain" eventually, and it's the least expensive system the home builder can install and lets face it they're out to make money. Because this system is based on the construction, there is no room for adjustment, over time the house settles and the gutter will hold water if the downspout is on the opposite side of the settling. You will be doing well if it lasts its 20 year projected life span.
5" gutter is not limited by a flange that's turned into the back but has a true flashing, an L-shaped strip of metal that is installed under the shingle and into the gutter to make sure the water does not get behind it, a major advantage over 4" because this system allows for a pitch or slant to insure proper drainage. 5"gutter will also carry a lot more water so the system is less likely to back up in heavy rain or large snow melts. It is available in over twenty baked on enamel finishes that are warranted for 40 years in aluminum or 20 years in steel. For most residential applications 5" gutter is now the standard and the one we install the most.
6'" gutter is primarily used for commercial applications but does have residential uses. If your house has a tile roof, or unusually large roof areas a 6" gutter may be what we would recommend. Obviously I have just scratched the surface when it comes to explaining how and what the different types of systems there are, if your interested in more info, please call or email firstname.lastname@example.org but this gives you a start. I believe knowledge is the key to all life's questions and the less you have to ASK for the better, I hope you will agree. Please call if you have further questions.