It's type is 'boost::int8_t' which guarantees it to have the size of 8 bits or 1 byte. Probably it is a typedef to some template instantiation.
If you ask which underlying type is used to implement it, than it is probably char. As far as I remember C++ standard guarantees a char to have the size of 1 byte. But anyway you can take a look at the implementation of boost::int8_t.