Is there a standard size screw for electric boxes in ceilings? Box is 25 years old and I am installing a new fan. 
UPDATE: a fan was removed from these boxes, however, the contractor did not keep the screws
Electrical – Screw specification for ceiling electric box
electrical
Related Solutions
There are a bunch of different styles of universal cross-bars, including ones that are designed to be mounted with a nipple. I'd just check your local hardware or big box store. Some examples:
It's fine in a single gang box if it meets fill reqs and the screws line up without modification. Cover plates may be difficult to find, but they exist.
Use electrical tape over the terminal screws if it makes you feel good. I do when the box is metal.
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Best Answer
Device (switch) boxes are 6-32. Junction boxes like that one are 8-32. Ground screws are 10-32.
Also, you could buy a ceiling hugger style fan instead of a stem mounted one, and fasten it directly to the ceiling with toggle bolts or similar fasteners, instead of just the box, since that box is most likely not rated for a fan.
Good luck and stay safe!