The earliest 'books' to have survived are scrolls (vellum, parchment or papyrus usually) and carved or painted stone. The earliest books in a form that we recognise today date from circa 5th Century. The overall dimensions of the page were limited by the natural processes of growth and the supply of raw materials. In the case of a manuscript on vellum (cow or deer skin), the number and size of pages is determined by such factors as : how many calves have local farmers produced and slaughtered? What age were they at time of slaughter and, consequently, how big had they grown? How healthy was the animal? What is the farmer charging per skin? How many can you afford? How many skins do you have the resources to prepare?
The manuscripts that survive can only give us a general idea of the overall page size and whether there was any significance in the measurements and proportions of the pages. Medieval manuscripts particularly have been rebound and trimmed more than once and hence any such information has been lost. One of the few manuscripts to survive in its original binding is the Cuthbert Gospel of St John. This doesn't give us an awful lot to go on.
So, a combination of practical and theological considerations is perhaps the best approach. The size of a calfskin will vary. You will need to leave sufficient margins to allow for trimming to a uniform size at the binding stage. A skin is also irregular in outline. You can clearly see where the head, tail and legs used to be and the skin is much tougher in these places. So your page size will need to avoid these areas. Another consideration is whether you want the skin of the spine of the animal to run horizontally or vertically across your page. Horizontal is more stable and the page will not move and cockle as much. This is, though,very wasteful of vellum.
As vellum is so expensive (and has always been), most people will probably squeeze the largest page or number of pages out of an average size skin that they can. It is then possible to tweak the vertical and horizontal dimensions to accord with a particular proportion or number symbolism. The most common numbers are given in the table below. We can't be sure that all of the meanings and symbolism are absolutely correct, but they are likely.
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