Raspberry Pi board uses M2.5 screws. Though you could size up to M3 with some drilling. Note that M3 screws are more common and they were used in PC floppy disk drive mounts.
20200311/DuckDuckGo raspberry pi m3 screw
20200311/https://raspberrypi.stackexchange.com/questions/23029/what-size-screws-does-the-raspberry-pi-board-use
Board thickness is 1.4 mm.
20200311/DuckDuckGo raspberry pi board thickness
20200311/https://www.element14.com/community/thread/57943/l/thickness-of-raspberrypi3-pcb
Raspberry Pi standoffs should be at least 11 mm board-to-board distance. Here are some parts that match those dimensions.
20200311/DuckDuckGo raspberry pi standoff height
20200311/https://www.pololu.com/product/1952
20200311/https://www.adafruit.com/product/2336
Here are some almost ideal parts on Digi-Key… slotted instead of Philips screws, but one millimeter short for Digi-Key standoffs. Use a paper/cardboard spacer I say.
20200311/https://www.digikey.com/product-detail/en/keystone-electronics/4707/36-4707-ND/4499300
20200311/https://www.digikey.com/product-detail/en/w-rth-elektronik/971100154/732-12917-ND/9488621
20200311/https://www.digikey.com/product-detail/en/keystone-electronics/29301/36-29301-ND/1532984