>>239
On amd64? Probably not much, you don't have to use the full registers (e.g. you have rax, eax, ax, al), you have movzx which can read a smaller sized number from memory and zero extend it one operation, etc. I'm not sure what happens on the microcode level, it might end up with an extra operation in some times, but it will likely just disappear in the pipeline and wait chains.
Other architectures are more tricky, you have to look into them. But for example, char is unsigned on arm, because armv3 didn't have ldrsb only ldrb which meant sign extension had to be done manually costing two more instructions. Things got better with armv4, but it looks like there are still some corner cases when unsigned char/short work better than signed in arm: https://stackoverflow.com/questions/3093669/why-unsigned-types-are-more-efficient-in-arm-cpu
For local variables just use int or size_t or int32/64, you're working with them, you don't want to convert them continuously back and forth, and hopefully they will be in registers anyway so you won't really save space either.
For non local variables probably go with the minimum size you need (but not bitfields), on a modernish CPU it's unlikely that integer conversion will be the bottleneck. Memory is slower than the CPU, so how much memory your code reads and how predictable your memory access patterns are usually more important.
And before you start optimising the shit out of your code, profile your app, because otherwise you'll likely optimize the wrong parts of it.