I have always seen elves potrayed as small dwarflike(in height) beings. If not smaller like "pixies". Did Tolkien change this image with LOTR? Is there a reason why his Elves are more like humans in terms of stature?

Personally, I think it's very cool to have them more human than small mysterious beings.

no, Tolkien didn't change the image, just borrowed. Celtic mythology maybe, could be Breton (spelled correctly) in origin, I forget.

Originally Tolkien wrote of dimunitive Elves and Gnomes and such but eventually changed them to Quendi and Noldor. He just kept the word "Elves" for convenience.