I've been to WDW in every month of the year except June.
In short, there's no golden answer to your question. Every month has pros and cons. It really depends on you, your tolerance/endurance, and very much the weather
For a start, hurricanes/blizzards often = canceled flights. It doesn't always matter where you are coming from. A blizzard anywhere might mean your outbound plane can't get to you.
January =short park hours+ many closed rides, especially all water rides. It will also be COLD in FL =long pants the whole time. (Jackets, double luggage! ) One year, we saw frost every morning in FL! The pools are not appealing when it's 50. (hot tubs are fun!) In Jan, we opt for compact resorts like AKL. Spring/fall is ideal time for outdoor hotels like Pop, IMO.
Summer, as you know can = thunderstorms. Do you stop in rain, or take advantage? We get a ton done after dark IN SUMMER. For us, summer is the better time, but much of the bonus is late night rides.
BUTTTTT.... the BEST thing I can tell you is that for us, length isn't all that important. We get nearly as much enjoyment from two day trips as nine day trips. Crowds and the Disney food start to grow old.
Oh...one last thought on picking a time of year. Going to WDW = missing whatever happens at home that week. Not much happens in most of January, but Summer = tons of things I hate to miss. summer trips make it hard to garden. If your home= snowy place, Jan can = coming home to a snow bound car.