In answer to this (though a little late...sorry)
Yes GS will stick to almost anything.
I'd use the left-over bits of resin you get on FW models (the stuff that looks like tank traps) failing that if your using Greenstuff for the main body of the model then it makes no difference what you use; greenstuff will cover and harden over almost anything. (Or you could just make balls of Greenstuff and cover it with Greenstuff, that works).
It should be smaller than a giant. If you make it too big it will look stupid. Plain and simple. The biggest you want it to be can be roughly measured by taking a 50mm base, get two cocktail sticks (or pencils or anything) put them against opposing edges of the base then lean them in to each other. Where they meet will be the groin/ waist/ belly of the model; any bigger and it will be too big.
On another note; well done on going for this! The more you do with Greenstuff the better you will get at it. It's ambitious for a first attempt at greenstuff so I wish you loads of luck with this and I hope it turns out how you want it.
On the subject of buying Greenstuff, I always get mine off ebay from theTrolltrader for roughly £7 for 36". Always done me proud.
Finally, for inspiration and a view of what you could achieve with years of practice why not check out the "god of greenstuff" Tupavko. I bow down and weep at his very awesomeness with this medium, he is my GS hero and a legend in my mind. Check his work (and weep oh ye mortals);
http://warhammer.org.uk/phpBB/viewtopic.php?f=4&t=106903&start=0Ah, you can't he's removed all the images! Better check him on Facebook then under Tupavko's Madness (I think)
G_H