Can anybody help me with the sizing of a ground plane for the ELT.

I wish to keep the size as small as possible and seem to recall reading
somewhere that a 5" CD has been used with some success and with little
discernable difference from using a larger copper/aluminium one as
recommended by the manufacturers.

John,

 

Verrry Interrrresting.....

 

Are you still in a fiberglass airplane?  If so, is the ELT broadcasting only on 121.5?  If so, an internally mounted Comm antenna should work without any ground plane - such as a foil "Z", etc. 

 

A vertically polarized antenna with a ground plane would seem to only work well if the crash left the plane in a normal flying/landing orientation.  Upside down or tail sticking up might not transmit enough signal ... maybe?

 

I am not familar with the antenna requirements for the higher frequency ELTs.