Will smaller sized dental implant in upper molar cause issues?

I had a traumatic fall & cracked a tooth & now have to get a dental implant in top left moler. I am getting zirconia implant & the doctor is an implant only specialist & said they are hard to get so I will have to do a smaller sized tooth there because that's the only option. Can this cause bite problems or chewing problems? And what does that mean? That the implant & tooth is smaller? And why is there limited zirconia implant availability in US? I see many companies online.