From the fire spread and prevention standpoint, the following guidelines are from IEEE 979 for liquid-filled transformers. You really should buy the standard, as I am technically infringing IEEE copyright by telling you this:
7571 liters or more of oil - 20 feet from building, if 20-50 feet from building, the exposed walls of the building should have a 2 hour fire barrier.
75 kVA or less - 10 feet from building.
76-333 kVA - 20 feet from building.
More than 333 kVA - 30 feet from building.
Add fire barriers if above minimums aren't maintained.
Large oil-filled transformers should be separated by at least 30 feet of clear space and/or a minimum 1 hour fire rated barrier.