How can I force the size of an int for debugging purposes?

You need an entire runtime environment including all the necessary libraries to share the ABI you're implementing.

If you want to run your 16-bit code on a 32-bit system, your most likely chance of success is to run it in a chroot that has a comparable runtime environment, possibly using qemu-user-static if you need ISA translation too. That said, I'm not sure that any of the platforms supported by QEMU has a 16-bit ABI.

It might be possible to write yourself a set of 16-bit shim libraries, backed by your platform's native libraries - but I suspect the effort would outweigh the benefit to you.

Note that for the specific case of running 32-bit x86 binaries on a 64-bit amd64 host, Linux kernels are often configured with dual-ABI support (you still need the appropriate 32-bit libraries, of course).

You can't, unless you find some very special compiler. It would break absolutely everything, including your printf call. The code generation in the 32-bit compiler might not even be able to produce the 16-bit arithmetic code as it is not commonly needed.

Have you considered using an emulator instead?