What is the typical vertical angle field of view for a PIR motion sensing light switch?
While it varies by the sensor, we can look at a good representative unit to get an idea

You are right that different sensors will be somewhat different with regard to their view angles, both in the horizontal and vertical planes.  A good sensor should provide numbers, or better yet, a diagram that specifies their viewing area in both axes though; this should either be given in a specification sheet/document, or provided by some sort of applications literature.  (There is a specification in NEMA WD-7 that governs this, and commercial-grade sensors such as a Lutron Maestro or a Legrand Wattstopper will comply with it, but there's no telling when it comes to random stuff on big-box shelves...)

For example, the diagram below, reproduced from page 59 of the Lutron Occupancy/Vacancy Sensor Design and Application Guide, provides an application-level specification for both horizontal and vertical sensor coverage for the Maestro PIR sensors at a given sensor installation height, provided the sensor has an unobstructed line of sight to the motion source.