Gutter is measured along the eaves. Sectional gutter comes in 10-foot lengths; seamless is rolled on site to whatever length the run needs, which is why it leaks less.
How many downspouts
The common rule is one downspout for every 30 to 40 feet of gutter. That is a starting point, not a specification — a steep roof shedding into a short gutter run needs more.
Undersized drainage shows up as gutters overflowing at the corners in heavy rain, which then dumps water against the foundation.
Five inch or six
Five-inch K-style gutter with 2x3 downspouts handles most houses. Six-inch with 3x4 downspouts carries roughly 40 percent more water and is worth it on steep roofs, large roof planes, or anywhere with intense summer storms.
The bigger size costs only marginally more in material and nothing extra in labour.
Hangers and slope
Hidden hangers screwed into the fascia every 24 inches is the current standard. Tighten that to 18 inches where snow and ice load the gutter.
Set a string line from the high end to the downspout at about a quarter inch of drop per 10 feet, and hang to the string.
Where the water goes
A downspout that discharges at the foundation is worse than no gutter at all — it concentrates the entire roof's water in one spot against the wall.
Extensions or buried drain lines should carry it at least four to six feet away, further on clay soils.
Sectional or seamless
Sectional gutter comes in ten foot pieces from a store and joins with connectors. Seamless gutter is rolled on site from a coil by a machine on a truck, in one continuous piece per run.
Every joint in a gutter is a future leak, so a seamless run has only the corners and the outlets as potential failure points. That is the main argument for it and it is a strong one.
Sectional is what you can actually install yourself in a weekend. If you are going that way, the joints are worth sealing properly rather than relying on the gasket alone.
Guards, and the honest view of them
Gutter guards reduce cleaning, they do not eliminate it. Fine debris still gets through most designs, and the guard itself collects material on top that has to be brushed off.
Mesh screens are the cheapest and work reasonably against leaves. Reverse-curve designs handle more but can overshoot in heavy rain, sending water straight over the front.
Under a pine, nothing works well — needles are the right size and shape to defeat almost every guard on the market. Factor that in before spending on a system.