Bill, you would have better luck with your question in a more appropriate forum, such as the Intel Fortran forums, or in Usenet comp.lang.fortran.
Secondly, you would need to give a more detailed description of what your needs are, since phrases such as "really large" have very little meaning.
The following page contains a number of links that you may examine to see if the available software there would work for you: http://crd-legacy.lbl.gov/~dhbailey/mpdist/ (considering integers as a subset of reals).
See also https://www.fortran.com/big_integer_module.f95 m for an integers-only solution. You will need to know the maximum integer beforehand with this module.
I thought LONG INTEGER would be self-explanatory.
I was referring to integers longer than the default length of integer(8)
Now one could get 15 byte integers by declaring variables as real(16).
but that is a sloppy and messy way of going about it.
These are used in cryptography and other branches of mathematics BTW.