Looking at recent photo's it appears that my spinaker is rather smaller than many other's even though it is supposedly a "full size" one, why are some spinnakers different in size and shape and is there an advantage to be had in one design over another?

How old is it?

Though the nominal size hasn't changed the last rule change went from a rule limiting lengths of leeches, foot and half-width height to one that calculates an area fairly accurately.  The previous rule effectively meant the largest size of spinny that would set well on reach and run was about 8m2, as against the present 10.

If you want a tall thin spinny cut fairly flat it will set well on a close reach but will not be good running.  A short fat one will be better for running (not the same as humans then).  Recent design has stabilised with a good compromise but if you want to do something different you can.

Andrew it's a 2006 Batt

2006 should be to todays' size.  The measured area should be written on one of the spinny clews and signed by the measurer.  There were some experimental shapes around in 2006.

If you look at your own spinny from your boat and compare with other boats viewed from your boat other people's spinnys always look bigger and fuller than your own.

Have a look at the picture in this link and I hope you will see what I mean.

http://s791.photobucket.com/albums/yy192/Smokin3496/Merlins%20at%20Earlswood%202010/

try this

Your spinny looks the right size to me.  James's was collapsing, it's possible his has longer leeches.

It should be full size if new in 2006, but it looks small to me. Has the size been written on it? If not and you'd like it measured to put your mind at rest give me a shout. I'm at Salcombe sat and sunday.