It's early defined in the docs. The machine also has a physical core count that is yours.

There is nothing stopping Amazon from having a machine that is double that config, and renting you half of it, however.

As soon as 32GB DIMMs drop enough in price, I would expect them to do exactly that. You can configure a Dell R930 with e7 Haswell CPUs online, if you want to double-check it. Not sure if the R930 can go to 8 CPUs (which the e7 Haswells CPUs support).

But as soon as they double the size of the machine there is no reason for them to not have an instance type that is the size of the machine minus the host layer.