I'm thinking of getting a furminator for my GR but how do I decide if I should get a medium or large. 

I got a small one for my beagle and my cats.  It was a better size for the cats than the dog but as she does not shed much it did the job ok.  Basically the smaller the size the longer it will take you and the more often you will have to empty it!

We had one from Petplanet. It is called a FurBeater and has a set of heads of different sizes. It's useful to us as we have large and small dogs here. But might suit you if you're not sure which size you need.

I have the medium for G.Rets but don't find it that useful.

That's interesting G.Rets what do you find is most useful, she's only just over 6 months but the one Ive got to use on her from 8 weeks old obviously isn't doing anything now.

You might find an Undercoat Rake - with a double row of teeth - a lot better.

I have one dog with a longish, thick double coat, and one short-haired with a double coat. The Furminator only really works on the shorthaired dog. It simply doesn't get right down to the roots of the longhaired dog's coat and barely draws anything.

The undercoat rake on the other hand, works for both and gets a great deal of coat out - with the satisfaction of knowing it's really penetrating the coat of the long haired one. I end up with just as much collected hair after a session with the undercoat rake as I do with the furminator - and there is the added bonus that the rake collects the hair, whereas the Furminator makes it fly and the very fine hair flying about always makes my nose itch.... Almost like a Strimmer versus a lawnmower which collects the grass!

I bought a large furminator for my goldies but have stopped using it as I ruined my boy's coat - took much too much undercoat out and now he looks scraggy!

I just use a wire slicker brush for the Goldens and a comb to get through the featherings. That has always worked for me and 12 Goldens over the years.