Although Wyverns can get to gargantuan proportions, my favorite description was Tanis' on the day of his impression. His dragon, Demios was very young at the time, and considerably smaller than most.
They come in a variety of different colors, including Black, White, Green, Emerald, Gold, Red, Blue, Grey, Silver, Brown, Terrain, Copper, Bronze, and Harlequin. They differ from other dragons, in that they have four legs, armor plating and wings.
Tanis had this to say when he met Demios.
The dragon was large enough to ride, but only just barely. Emerald green in color, slim and fairly long, she stood about seven feet high and I estimated her weight at about six hundred pounds. With a low rumbling sound, she spread her wings to about three quarters open, where they stayed and quivered with every breath taken. Yellow green eyes stared at me with a look that I could not read. Her tail was long and slender and sloped down to an end point that was unremarkable to what I thought dragon tails required. No barbs, ridges or any other adornment that one envisions on every dragon tail. Just smooth reptilian scales from stem to stern. Her head had the typical western dragon shape, if maybe not just a bit toward the triangular wedge look. The ears looked like triple paneled accordion fan blades, but at the moment were fully extended and held back along her skull in the opened position. The underbelly was also green but several shades lighter than along her back. Her feet had the same number of toes that we have fingers on our hands, just longer, obviously reptilian and equipped with talons on the end of each toe on all four feet. She was thin, elongated and muscled but in a more athletic way. She was young and powerful, and I was speechless and in awe. Smoke began to trickle out of her nostrils and I took that as a bad sign. She turned her head and I could clearly see upraised cheek bones that ran diagonally from her jaw, a jaw that had more teeth than any creature had the right to own.