Roof area, not gutter length, drives the sizing
The number that actually determines gutter and downspout size is the square footage of roof draining into that particular run, not the length of the gutter itself. A short gutter catching runoff from a large, steep roof section can need more downspout capacity than a long gutter draining a small, low-pitched addition roof. We calculate roof area for each drainage section separately rather than applying one downspout count to the whole house.
This is the step that gets skipped most often on a fast production-home install, where every house on a subdivision gets the same standard gutter and downspout layout regardless of how the actual roof planes are shaped.
Standard sizes and when to go bigger
Most residential gutter here runs five inch or six inch profile, with six inch handling meaningfully more volume during a heavy downpour. A steep roof, a large roof area feeding one gutter run, or a history of overflow on the current system are all reasons to step up to six inch rather than replacing like for like with the undersized original.
Downspout size matters as much as gutter size. A five inch gutter paired with an undersized two by three downspout will still overflow in a hard rain even though the gutter itself has plenty of capacity, because the bottleneck is at the downspout, not the trough.
Downspout placement and count
More downspouts spaced closer together move water faster than fewer downspouts at the ends of a long run, and on a longer roofline we often add a downspout in the middle rather than forcing all the water from both directions to travel the full length of the gutter before it can drain. This matters most during the kind of sudden, heavy summer downpour the Red River Valley gets, where a gutter with too few downspouts overflows within minutes even if it is completely clean.
What undersizing looks like in practice
The tell for an undersized system is overflow that happens specifically during heavy rain rather than gradually over a clogged season, and overflow that happens even right after a cleaning. If a gutter overflows in a downpour but drains fine within an hour afterward, sizing is usually the real issue rather than debris. We check this during any gutter inspection before recommending cleaning, guards, or replacement, since fixing the wrong problem does not stop the overflow.