The thing to remember about division size is that it generally helps dictate how long the season will be. The best weekly format I've seen is "one match per week, two weeks per match".
Players need to be in similar time zones (or have similar preferred windows; maybe I am PST but I want to play at 6am every day, so I fit better in the Europe division) to facilitate matches getting done. Then, you have to be prepared to hand out forfeits, make judgment calls on who is failing to schedule, etc. It can get pretty stressful! So those are the two major factors in getting through the season; proper divisions based on preferred playtime and doing the hard TO job of making those tough calls.
In the last season, I split up these duties (overseeing match conversations, handling disputes) among four people in relevant time zones so that those divisions would each have an accessible TO. It doesn't make much sense for me to be the Asia TO because I'm in California, so it would be tough to actually talk to me if somebody had a dispute that needed solving immediately (thanks again to @CloudCuckooCountry @Corroyeur @Bomber678 for all of your help).
So depending on how many sign-ups you get overall, it really helps to split up the duties so that you aren't one person handling all of the match conversations yourself.
Remember also that 10 person divisions mean a 9 game, 9 week season, but an 11 person division means an 11 week season (5 matches per week, one player has a bye and each player plays 10 games). So the higher you go above 10 per division, the longer the season gets especially if there are byes involved.
The last consideration is how to determine the playoffs. You need to keep participation high so that someone who is 0-5 still feels like it's worth showing up to play. If there's no chance to get into playoffs at that point, then you might end up with a few 0-10ers in each division and you have a season with a ton of forfeits.
It's tough to find the balance!