A flexible calendar view implemented using a UICollectionView. Check out the demo app. You almost certainly want to subclass the cell class (example in the demo app) and probably the header view class (not done in the demo app).
Thanks to UICollectionView it supports any size and also allows resizing. If you set non white/clear cell backgrounds or borders however, you will want to be careful with your sizes to avoid inconsistent gaps between days. Also, if you change the width during rotation, animation is a bit laggy especially on iOS 6.
A major caveat at the moment is that the headers are very sparsely documented.