I think the IBM 7030 Stretch CPU (from 1964) would be a likely contender for being one of the first 64-bit CPUs.
Otherwise most modern CPUs would be labeled as either 256-bit or 512-bit.
These days we generally label CPUs based on the width of the general purpose registers (though, it gets messy with things like the 68000). I personally suspect we won't ever see GPRs wider than 64 bits.