Though 5- and 6-inch gutters are industry standards, several factors can determine whether your home might need a larger gutter.
The terms "5-inch" and "6-inch" refer to the width across the top of the gutter. And in most cases, these sizes are sufficient to drain water away from your house. But two factors might argue for a 7-inch gutter. First, you may live in an area of the country that's subject to steady drenching rains or sudden torrential downpours.
Second, in recent years many newer homes have been designed with multiple rooflines and numerous roof valleys. Roofs with steep pitches have likewise become more common. As the housing market has become more competitive, the simple rectangular ranch has given way to the McMansion. American homes have also steadily gotten bigger over the past generation—and that means more roof area that must be drained.
Determining the optimal gutter size for your home—as well as the required number and size of the downspouts—is where a professional installer can be a help. They can consult the standard industry manual, published by the Sheet Metal and Air Conditioning Contractors National Association (SMACNA), that details expected rainfalls for each area of the country. They can calculate these rainfalls against the square footage and slope of your roof. They can determine the weight of water that a given section of gutter must hold, and thus the number and spacing of the hangers needed to hold up the gutter.
Likewise, the installer can ensure the gutter system is properly installed so that backups, blockages, ice damming, and other problems don't skew the calculations and cause overflows anyway. And finally, if a larger gutter size and extra downspouts are needed, the installer can find ways to balance function and appearance—perhaps using colors that blend in with your home—so that the gutters don't dominate the exterior of your house.