tl;dr: Those models that have holes in the PCB are made for M2.5 (or UNC 3-48) screws and a length fitting to the enclosure or respective counterpart.
Pi 1 B+ contains four M2.5 mounting holes - supposedly drilled to 2.75 +/- 0.05 mm. This information is taken from the "Raspberry Pi B+ Mechanical Schematic" (official drawing by J. Adams, 07/03/2014, to be found at RS Components).
Pi 1 B (rev 2) contains two M2.5 mounting holes (as derived from own measurements, diameter is roughly 2.8 mm). See also http://www.raspberrypi.org/upcoming-board-revision/ that describes the existance of drilled holes of 2.9 mm diameter for M2.5 screws.
Pi 1 B (rev 1) does not have any mounting holes.
Pi 1 A shares the board layout with the Pi 1 B (suggested by TomG and supported by available pictures).
Addendum (per jmb's comment, thanks!):
https://www.raspberrypi.org/documentation/hardware/raspberrypi/mechanical/ has now mechanical drawings for the Pi 3, the Zero (v1.2), Pi 1B+ available. They show the size and position of the holes as well as board outline and position of connector. These models all share the 2.75 mm holes.