need help in UHF near-field loop antenna

hi

I'm using the CST MWS to simulate the UHF RFID near-field loop antenna, and i modelized the antenna as following. But i don't know what size of the loop is suitable? How can i get the right geometry of this loop?

For this kind of antenna, i used "discrete port" as the excitation. The H-field is monitored as following.

Any suggestion to improve the performance of this loop antenna?

Thanks in advance!

FC