The ideal tamper should be slightly smaller than the inside diameter of the portafilter basket so that the tamper does not bind while compressing grounds - essentially there should be a little extra space between tamper and basket.

What should I look for in a tamper?

*Right Weight and Right Diameter. *Easy to tamp and not harmful for you hand if you tamp many times day. If you make espresso shots frequently， Get one right coffee tamper is very important for you. A good tamper means lightweight metal and the size of Portafilter is correct for your tamping.

Does tamper size matter?

For tampers, size DOES matter. Every espresso machine has a portafilter. ... Most commercial espresso machines will have portafilters that require tamper sizes of 58mm, but there are also some that may be a bit smaller than this too. Some of of the other sizes out there include 49mm, 51mm, 53mm, and 57mm.