More unsolicited advice to younger colleagues:
If you intend to enter a competition for student composers, the format of your scores should follow the secret rules. In my own student days, back between the Late Cretaceous and the Early Volcker Administration, the prime directive among the secret rules was that scores submitted to competitions had to be printed in that particularly inelegant process, associated with engineering and architecture, involving inkpens, knives, translucent "onionskin" paper, ultraviolet light and ammonia vapours, which carried the somewhat reptilian name "OzlalidTM", as use of this medium was considered by juries to be "professional." Scores made via photocopying, no matter how high the quality of paper, printing, or formatting, were frequently simply sieved out of the selection in the initial round of judging as insufficiently- if not "un-professional". It didn't matter that OzalidTMs, as an emblem of professionalism were ugly, expensive, and rarely in real use outside of a few commercial music centers, it was just one of those secret signs or handshakes between those in the know.
OzalidTMs have, in professional, both real and academically imagined, new music usage, blessedly gone the way of Ozraptor subotaii, but newer forms of a prescribed and fictional professionalism have just as rigidly taken their place among the competition set. So, if you decide to go the competition route, following something like these rules by a known serial competition judge would probably be a first step in gaming the competition. It doesn't matter that these rules are a mixed bag with limited reference to actual professional practice, these are rules reflecting actual judging preferences. Some of them are common sense — like using cues in parts (and, while you're at it, don't forget to notate important cues in the score to help the composer help the players) or laying out the parts with good page turns —, some of them are reasonable suggestions — double bars at tempo changes can be useful —, and some of them are complete cattle scat — like forbidding a landscape orientation or measure numbers at regular intervals of 5 or 10 measures, both of which may in fact be optimal solutions in particular musical contexts.
So much for competitions. What about real life? Players increasingly like to have their materials immediately rather than wait for the post and one of the blessings of more recent technologies is that scores can be made and stored in electronic formats which can be emailed or downloaded on demand and then be printed out on by the end user on whatever paper format is locally available and personally optimal. Specifying an odd page size for scores like ottavo or 10 x 13 inches is both anachronistic and parochial. In real life practice, real professional musicians will rearrange, cut, paste, fold and otherwise mutilate the performance materials as they most usefully see fit. They will use the printer and copying technologies and formats at hand, and no matter what you do or prescribe, these will frequently be off the shelf photocopiers and laser or inkjet printers, printed to stock A4 or letter-sized pages. It may be wise to be prepared for this. Many new music ensembles prefer to play from scores rather than parts, single players in these ensembles are mostly well practiced in using multiple stands, so landscape formats are not a difficulty (in fact, if the music involves long, continuous, lines, a landscape format is often optimal). My own experience has been that players in new music ensembles so prefer to arrange their own parts and scores; my own publisher offers both in loose page as well as bound formats, and the majority of orders have, in fact, been for loose pages.
Finally, I think that this is not a matter of rules, but rather a matter of sensible and sensitive negotiation and optimization, in which the composer is simultaneously engaged with her or his own compositional ideal and the means towards a practical, efficient, and qualitatively satisfactory realization by members of the community of musicians. Conventions and suggestions derived from the experience of similar musical situations may well prove optimal at times, but new music, unless it foregoes innovation for the quietude of the conservatory and competition class, is bound in other times to come to loggerheads with even the most familiar and unobjectionable convention.