Hermaeus Mora himself refers to Apocrypha as being endless in extent.
Hermaeus Mora: I know you, champion. The Oghma Infinium was only the beginning. This is Apocrypha, where all knowledge is hoarded. Sate your thirst for knowledge in the endless stacks of my library. If you tire of your search, read your book again to return to your mortal life. For a time. The lure of Apocrypha will call you back. It is your fate.
Vestige: Tell me about Apocrypha.
Hermaeus Mora: I created and rule over Apocrypha, my endless repository of knowledge in the vastness of Oblivion. Every secret known to mortal and immortal resides here, as well as many known only to me. It is a realm of fate, time, and wisdom.