In practice, how much of my code is going to be run on an 8-bit CPU? Zero. I'd rather have the clarity and consistent behavior of explicitly typed ints than pre-optimize for something that will almost certainly never happen.
Silently increasing the size of an int is more likely to break things than it'll be helpful. If you really want an int to be 32 bit on a 32 bit machine and 64 bit on a 64 bit machine, make it explicit (e.g. intptr_t for pointers)