optimum print resolution is more than simply pixels and ppi, it has to do with the total resolution of the lens/sensor combo in line pairs per mm, which factors in the f-stop and sensor size, but only tells you a best-case scenario since it doesn't account for camera shake.
...but I doubt you're making a fine art or competition print, so print at whatever size you like. small ppi count will be masked by a longer viewing distance anyway.
tip: don't center the subject unless there's a good reason to.