See rule R89
<R89> In addition to the pneumatic cylinders provided in the Kit Of Parts and the “free” pneumatic
cylinders available for order through the Free Pneumatic Components Order Form,
additional air cylinders or rotary actuators may be used.
All cylinders, regardless of source,
must be identical to those listed on the Free Pneumatic Components Order Form (e.g. same
part numbers). Any additional air cylinders must source from Bimba or Parker Hannifin, or
be recovered from prior year FIRST Kit Of Parts.
The emphasis above is mine.
Here is the free order form:
http://www.bimba.com/FirstFree.aspx
Note that the largest cylinder you can purchase is 24" stroke 2" bore.