This RFID Reader Module (Serial version only) comes with two 54x85mm Rectangle Tags.

Designed in cooperation with Grand Idea Studio, the Parallax Radio Frequency Identification (RFID) Reader Module is the first low-cost solution to read passive RFID transponder tags. The RFID Reader Module can be used in a wide variety of hobbyist and commercial applications, including access control, automatic identification, robotics, navigation, inventory tracking, payment systems, and car immobilization.
There are a variety of transponder tags that come in different packages. Each tag has a specific range that is within 10% of the given distance for each type of tag. The reason for the 10% is due to environmental conditions and RFID modules.

Features:
Low-cost method for reading passive RFID EM4100 family transponder tags
2400 baud serial interface
Enable input allows module to be enabled/disabled by software
Bi-color LED for visual indication of status

Compatible tags:
54x85mm Rectangle Tag ISO Card: 6.3cm (2.5") +/- 10%
50mm Round Tag World Tag 50mm: 6.8cm (2.7") +/- 10%
Blue Eye Key Fob
25mm Disk Tag

Key Specifications:
Power requirements: 4.5 to 5.5 VDC
Communication: Serial at 2400 baud (8N1)
Dimensions: 2.45 x 6.25 x 0.22 in (62.2 x 82.5 x 5.57 mm)
Operating temp range: -40 to +185 °F (-40 to +85 °

Note: This device can be connected to a PC serial port using a MAX232 line driver. The circuit isn't supported by Parallax, but it's possible to make this connection with a few dollars of parts or by using the USB version of the reader.