The CCLF tube assemblies are made up of a U shaped reflector with 2ea individual tubes inside. The entire assembly is 457mm long and 7.6mm wide. There are no part numbers on the assembly however there is a S on the end with the wires attached and an 18 on the other end.

The 2 CCFL tubes are held together with a figure 8 rubber band in the middle of the reflector so both tubes must come out at the same time.

The individual tubes are 448mm long (excluding the terminals - standard way to measure is end of glass to end of glass) and are 2.4mm diameter.

The CCFL are soldered to the wires so not a simple plug in replacement

The CCFL are EXTREMELY fragile so ensure you dont break them when installing them.

The CCFL reflectors are a tight fit over the sides of the perspex sheet and the other layers of plastic. Patience and a plastic spudger or pry tool should get you there

The closest I can find to my CCFL measurements are either the 446mm x 2.4mm or the 450mm x 2.4mm so I'd suggest the 446mm is used

See the Removing CCFL from iMac 20in LCD CCFL assembly guide for details