How big can the wyvern dragon get?

It is the biggest of all dragons. : False

Wyvern Dragons aren't actually as powerful as some may think, they're actually quite low in the food chain. Unlike European and Phelera Dragons, Wyvern Dragons can't control their size freely. On average, a male Wyvern may get to about 30ft. from head to tail. An average female Wyvern may get to about 33ft. from head to tail.

The Wyvern is in fact one of the smallest species of dragons, according to the DUngeons and Dragons wiki and several other dragon website references (Lady Gryphons Mythical Realm, Wikipedia, The Circle of the Dragon) and at that, not a true dragon at all. Unfortunately while there are many references to it being much smaller than a true dragon, as of yet, I haven't been able to find an exact length or wingspan either. So unfortunately, we seem to be stuck with a very vague range of smaller than a true dragon, but larger than a Fae Dragon. If I can find a more definite answer, I will be sure to update this answer and provide the reference where I found the answer.

Well, that didn't take to long. I have found a website which lists the length of a Wyvern as up to 35 feet. It is called the Monstrous Manual.