With RFID UHF tags, there is a general rule of the thumb:  The larger the tag, the longer the read range.  This is generally true when comparing tags from the same time period.  As with any technology, performance improves while the size decreases over time.  Therefore, a 4' x 4" tag from 2005 will generally not perform as well as a 2" x 2" tag from 2008.

A critical component is the antenna design.  Unfortunately, there is no one best antenna design.  The optimal design is dependent on the application which includes form factor, size limitations, dielectric characteristics of the material to which it is being attached, frequency optimization, etc.
Various RFID tag antenna types

Another general rule of thumb is that a single dipole tag properly lined up with a linear antenna will have a longer read range than a dual dipole tag with a circular antenna.  In my earlier post about RFID antennas, I discussed the difference betwen linear and circular antennas.

Although this is a high-level view, the basic RFID tag factors which affect read range are:
     1.  size
     2.  antenna design
     3.  type

The next in this series will discuss how the item itself affects the read range.