This bird, right here in this link, is a phoenix. Or so the character list tells.
And my first thought of it was that it was awfully small.
And my second thought was humm, does the primary source actually speak of its size? I reflected on William Shakespeare.
A little research later, I found that most of the ancient writings spoke of it as being like an eagle or a peacock -- it may originally have been some kind of heron -- so I had reasons to remember it as a large bird besides other fantasists using it as large.
Then Shakespeare's "The Phoenix and the Turtle" features the phoenix as the turtle dove's bride, so some historical flexibility means it's not inaccurate, if a bit odd.
Then, even without that, swallowing alterations for the story's sake can keep the thing going, and avoids making you look foolish. I still remember the critiquer who could not handle that a dragon was small enough to ride a horse, even though many Eastern Europe fairy tales feature dragons that small.