I don't know that there is a record for the largest boil, but I've seen them as big as tennis balls. When a large boil ruptures or is incised and drained by a clinician it leaves a large void in the body. Over time this will eventually fill in, sometimes to the level of the remaining uninfected skin, and sometimes there is a little "dent" where the boil was. It is all determined by the person's own recuperative powers, their age, what cells were damaged, etc.
A boil is an infection in a hair follicle or sweat gland by the bacterium Staphylococcus aureus. Since staph is everywhere, it can get into the skin at a hair follicle or any tiny cut or abrasion. That means that boils can recur in an adjacent hair, or elsewhere in the area, or anywhere on the body that the germ can get in. Some people are staph carriers, so it is beneficial if everyone in the family uses a medication called mupirocin (Bactroban®) in their nostrils twice a day for a week to eradicate the staph. It is also useful if one has a problem with staph to bathe with antibacterial soap and to make sure to wash the hands frequently. Here is some more information on boils.