When I used to organize community tourneys I just messaged logress with theme time and other details after I made a thread about it.
Reason you message him are rewards for the winners. In case he agrees. + he always done a front page announcement.
Tourney you hold in training and it usually lasts 2 - 3 or even 4 hours, at least that's the duration of my tourneys.
Each of my tourneys had some weird theme.
I won't do one. Holidays for many and also my free time ain't fixed. + no mood either, sorry.
I can handle any amount of players. But any number over 64 means slow tourney.
It's a chaos if you can't script the rules inside the arena.