750 ml wine bottles ARE acceptable this year. However, keep in mind that a single 12 US fl. oz. beer bottle is approximately 355 ml, so a single 750 ml wine bottle is just slightly more than two beer bottles' net volume. In order to ensure that you have enough mead on hand to survive the prelim judging, then a possible second round (to select best of category), then possibly Best of Show and/or subsequent tasting for the Pro-Am, the minimum you should send per entry is two wine bottles if your mead is still. For sparkling meads I would recommend sending three bottles, since you don't want your mead de-gassing between rounds and losing your intended degree of sparkle as the day wears on.
And yes, to be completely comprehensive about it, one 750ml plus one 375ml is slightly more than three beer bottles' volume so if you sent in a one-and-one entry like that, you'd be supplying enough mead to "run the gauntlet" of rounds, but if you're that protective of your mead then I'd recommend you just send it in beer bottles to begin with - it'll be much easier for us to handle that way! ;D
Just a note here.
Sending mismatched entry sizes presents us with a number of issues. Storage of entries becomes complicated with mismatched bottle sizes since we try to minimize the footprint of our storage area by keeping like-sized bottles together and as a result the potential for loss his higher. At the competition there is a higher potential that your mead will sit through several other flights of judging (hours) after being opened before it gets back to the judging tables for the BoS round if it makes it that far. Another key factor is, that if one of those bottles gets broken, leaks, or has an issue, you're done! You won't make it past the first round.
If you already have your mead in wine bottles, send two and you should be covered unless one breaks during shipment.
I also want to say that Wayne is a very patient and dedicated guy and he already goes way out of his way to do everything he can to accommodate really questionable entries that we have received in the past. I ask that you folks bear in mind the really daunting task that Wayne faces every year of going through six hundred bottles or more, by hand, after work, on his own time in order to verify, identify, register and account for your entries. He is a star in every sense of the word.
It is just simpler, more secure and overall more conducive to the flow of the competition to go with three like sized and shaped bottles of 10oz or more, or 4 bottles under 10oz. Color, closures, raised lettering and shape isn't an issue this year.
Cheers,
Oskaar