this is entirely my understanding, so i will not vouch for any of it
first, the personalized menus are available to anyone, even those who don't get a diver sign. you can just call the restaurant and go through the menu to speak to either Debbi or Meredith about it. As far as limits go, Debbi has told me there have been guests who have sent her 12 pictures and she'll do her best to make it work. they prefer all this at least a couple of days in advance, although she has said she's even done it same day if necessary (but we do need to be fair, especially since it's free).
as far as the diver sign goes, it's limited to the number per night for a couple of reasons. first, it would become less special if everyone in the restaurant got their own sign.. second, out of respect to the fish. with that in mind, it's best to confirm it as soon as you have an ADR. i suspect if you're too early (if there's such a thing), you'll be told as such. i was able to confirm one about 5 weeks out, but had to have my ADR time changed to accomodate it.
as far as length goes, it's an 8.5x11 sheet of paper that's laminated. the less the message, the bigger it can be and the easier it can be seen.