irb(main):001:0> RUBY_DESCRIPTION
=> "jruby 1.7.8 (1.9.3p392) 2013-11-14 0ce429e on Java HotSpot(TM) 64-Bit Server VM 1.8.0-ea-b82 +indy [Windows 7-amd64]"
irb(main):003:0> FFI::TypeDefs[:size_t]
=> #<FFI::Type::Builtin:UINT size=4 alignment=4>
irb(main):004:0> FFI::Platform::NAME
=> "x86_64-windows"
According to all the code I can see it should be ulong_long?