I think the correct answer is 'at least one page'. (because if it was zero, it wouldn't be a manga adaptation)
There is no set limit on how much content one must have in order to get an adaption. The deciding factor is instead decided by the production company, sponsors and any other involved parties that make up the committee who decide which animations to produce. It does help the management board decide on a series if it the series has significant existing fan interest from the manga.
Now, while I can't think of any one-page manga that have been adapted, it is perfectly possible if the committee agrees to it - There will just be more effort put into planning around this.
This blog post explains further about the anime production process:
This process depends on who’s pushing for an idea and who is backing it up, it can be animation studios themselves along with sponsors, but many anime are adaptations of manga or light novels, in which case, publishers front costs (including the costs of having it shown on TV stations). The production company (e.g Aniplex) gathers staff, sponsors, and looks at advertisement and merchandise. While many people describe studios as being cheap, only around half the budget is often given to the anime studio, with the rest going to broadcasters and other contributing companies.
It is also worthy of note that many mangas themselves are adaptations from light novels or visual novels or may have other spin-off content which adds to the content that the studios can work with.
The content range of which adaptations do come to life can vary significantly. This is to do with chapter sizes, author preference, committee requests, etc. Every show will have different requirements and differing ease of adaptation.
For example; Chihayafuru's first season (of 13 episodes) covers 40 chapters & Claymore with 22 episodes covers roughly 60. Hikari lychee club is a manga adaption, but is very different in style and story.
This is probably a bit of a disappointing answer, but there is no minimal content requirement and it is something that differs from company to company.