Am I right in thinking that dogs don't relate to size when wanting to say hello to another dog.  I ask this because whilst walking my very good natured GSD, I constantly get told by smaller breed owners that my dog cannot say hello to theirs because they only have a small dog.

Well socialised dogs can get on perfectly well with dogs of all other sizes. There's no reason why your GSD shouldn't say hello to even the tiniest dog - unless its owners think their little one might bite your dog.

Funnily enough I only put on another thread how well behaved my larger dog was with two little dogs yesterday.  My dog has been iffy in the past with some (not all) dogs she doesn't know but on the whole I would say they have been larger dogs but that's because it's usually larger dogs that we see.  I think this is one of those topics where size really does not matter

Sometimes big dogs can "forget themselves" when playing with smaller dogs and you could only imagine a GSD paw bopping a small dog in play.  Althou my grandparents had a huge boxer dog who became best friends with a little yorkie and would be ever so gentle with the yorkie. 

If dogs are socilised well with all dogs of all sizes then they are normally fine   Though IME and IMO little dogs are alot more snappy.

I realized that it all depends on the situation. I have a Chihuahua that is sweet with every dog no matter the size. He is very well behaved and hardly ever barks at anyone. Unfortunately I've had many other dogs barking at him (small, but mostly big ones for some reason) and at that point he gets a bit scared. I let him sniff all the dogs but ONLY if those dogs are under control on the leash..... because believe me, I will not risk my 1,6kg Chi being accidentally killed by some 30kg dog that is getting too excited and trying to play with him. I just had a massive Labrador (I love the breed, I had a lovely Lab girl myself ) running towards my Billy and I had to lift him up or he'd probably be squashed in a sec. The woman was running after her dog and showting "he's soooo friendly, he won't do anything". I told her that I don't care if the dog is friendly or not, because just one wrong step and my little one is dead. The dog started jumping on me and made me all dirty. My Billy was freaking out (I really hate that, because I spent months of training and a lot of money on socializing him) and the woman didn't even have the decency to put him on the leash !!

I have a yorkie and a boxer cross and the yorkie is WAY more Snappy.  No matter how much socialising hes had.  Hes walked with a Lab called Zac thats now scared of my yorkie which is awful.   Where as my boxer cross is very chilled out.  I think Arthurs got small man syndrome

Louis - my big male Groenendael - is completely cowed by my neighbours wee mini dachshund - when we go on our shared walks he always stays a respectful couple of paces behind her ( not easy given the difference in leg length :D) she always gets to carry the ball/frisbee etc and can floor him with one look LOL !

Yvonne

I had a cairn terrier once that played very happily with dogs his own size or smaller but he was terrible with dogs bigger than himself and he was the one that started the trouble, he had many big dogs cowering. I put it down to having a chip on his shoulder about being small!!
Amos