Typically you want the same or larger openings downstream to prevent back up at full flow. If you find a cure for the oak tree string stuff PLEASE let me know. I have yet to find a reliable "filter" as they just make things clog sooner and everything overruns the downspout anyway. Regular cleaning is the best solution (At least mine are reasonably accessible).

FWIW I put new seamless aluminum gutters a few years back because the roof was done and things really didn't change over my existing wooden gutters.

At one time I tried gutter guards (expanded metal type screens) and in my opinion it was more difficult to get the tangles out of the screens.

Good luck

i have been looking for a solution too. So far the best has been a leaf blower. I did check out other houses that do have the 3x4 downs and they had clogs from those stupid oak debris.

Seamless gutters are expensive...downspouts are very cheap. I would not replace the gutters unless they are shot.

I would try to widen the opening and go with bigger down spouts. Shouldn't be expensive so why not. Keep bends to a minimum and make them gradual to prevent clogs.

Are you doing this yourself of paying a company to do it?

Are you putting in leaf protection at the same time?

Most folks just want to pay a professional to do it as it is a bit difficult work and can be dangerous.

Once you get past the job cost minimums, you may not find that the expense of the gutters is that much.

Can you post up a picture?