This all applies to Ruby 2.1. In some cases a setting is not available in 2.0, this is noted. There is also a different with 1.9, 1.8, and REE --- these are not noted.
All the relevant code is in https://github.com/ruby/ruby/blob/trunk/gc.c
The number of heap slots to start out with. This should be set high enough so that your app has enough or almost enough memory after loading so that it doesn't have to allocate more memory on the first request (althogh this probably isn't such a big deal for most apps).
(todo: figure out how big a slot is. i think the answer can be infered from this code.)