Personally I think 9v9 is utter rank. 4v4, 5v5, 7v7 at least follow a certain shape (leads nicely into a 433 or 4231). What does 9v9 do?
Been through this pathway a few times, had prepared the kids well, and they took to it well. Tried 9s and hated it.
Biggest problems going to 11s are:
Goalies have no experience kicking the ball and the size of the goals
At 11s you are adding on a back 4. Defenders have no experience of defending as a unit, headering the ball, balancing, etc...
The offside rule.
I personally, through experience, see no benefit in 9s. Also, re squad size, 2x 8player squads at 7aside let you form a nice 11s squad.
If doing 9s, do some players get less time or do you increase size of 2 squads then have to lose a few for 11s again?