The main aim is to create a print quality paperback book - the kind of thing that sits on a bookshelf with all the other paperbacks without being bullied because it doesn't have a penguin on the spine.
Helpfully, many moons ago, all the paperbacks got together in a forest clearing and had a huge argument about size. Eventually, here in the UK, they reached something close to an agreement. There is variation for small presses, US publishers and large volumes, but on the whole 130mm by 190mm has become standard*. I don't know who decided this, or how the regular width became came to be 130mm instead of 125mm or 135mm, but I'm glad it happened. It makes things easier all-round, from printing to designing to shelf-stacking.
Oh, on a related topic, does anyone know who decided that loaves of bread were 800g? I've always wanted to know ('always' may not, strictly speaking, be true). In my head it's down to a sinister cabal of international mega-bakers meeting in a secret bunker under the Tower of London, but I appreciate that this isn't very likely. The Tower of London is so old hat. These days all the cool conspiracies happen on secret desert installations, or in mysterious shady corners of the internet.
Anyway, after the page size came book length. With thirty-one writers, plus scrappy bits like an intro, prelims (more on these later) and author biographies, ultra-narrow isn't an option. It's all very well trying to catch agents' attention, but chucking them a couple of brief paragraphs by each writer is going to be an unsatisfying experience for all involved. Also, since we want people to buy the thing, a bit of heft might make it a bit more tempting.
On the other hand, we don't want a doorstop, or something that's more useful for building with than reading. Sticking with a rough estimate for now, a length of around 350 pages seemed like a reasonable compromise: not something you need a forklift truck to pick up, but not something skinny as a Topman trouser-leg. That's roughly the size of an average novel, which again will help this look like a professional job.
This might all seem vague and pointless, but having basic estimates like these is important for the detailed planning that follows. With an approximate idea of the size and shape you can start gathering print estimates (and thus leap into the mysterious land of budgeting), you can start putting together some page templates, and you can work out what you need to request from authors, and, even more importantly, when you want it to arrive.
Believe it or not, those will be the next three blog posts. Unless I forget, or get distracted, or have a better idea. Because that's the other thing this publishing lark teaches you pretty sharpish: sometimes you need to be ready to change things as you go.
* There were going to be some pretty pictures demonstrating this, but the camera on my telephone is sulking, so you'll just have to believe me. Or look at your own shelves, I suppose. Either way I'm disappointed. I'd gone and arranged my bookshelf in colour order and everything.