I have small (3x4) cedar gutters - I just replaced the old ones. When I had the roof re-done, I specified that the roofing MUST allow for easy cleaning of the gutters. The old roof overlapped so there was barely room to get a finger in some places. They made special drip edges which leave the gutter fully open. I made a air blast nozzle with a 10 foot length of copper pipe, a ball valve, and 2 threaded PVC "L"s to point the nozzle any direction. I just walk under the gutter sliding the nozzle along it to blast everything out. The advantage here is the wood gutters which have no spikes or other obstructions to block the nozzle or catch pine needles. I do need to treat them with shake oil every few years.
They do make special extenders for leaf blowers for blowing out gutters.
The rain here in Seattle is usually so light that 2" downspouts are plenty except for maybe 2 or 3 storms a year. Then they overflow a little in a couple spaces - no problem. It almost nevers rains here like the spring rains back east.
You do need to keep the debris from washing down the downspouts if it goes into a drain field.
Bob