I have zirconia caps on my front teeth. One is longer than other. Can the dentist shave it down?

If the crowns are porcelain and zirconia crowns they should be able to adjust them without any issue as long as there is not a significant amount that needs to be reduced. If they are full zirconia crowns then they cannot be adjusted much if any. Hope this helps. 

yes zirconia crowns can be shortened.  The questionthat needs to be answered is whether the tooth that appearslonger is the correct length and the other tooth is too short.Before I go about shortening a tooth I would look and seewhich length is ideal for you.  If the other tooth is too shortthen shortening the longer tooth may take away from an idealsituation.  You may need to replace the shorter crown if youare a perfectionist.

Zirconia crowns can be recontoured if they are uneven in length. It is important to do this carefully and with lots of water to keep the restoration from heating up.