Shadow plane
The baby owl's body was only slightly larger than Lex's own, but that did not mean it was not a good fit. Its body carried a hidden strength that had been tucked in, to keep it from detection while it searched for enemies. But that could not hide it from Lex, who was beginning to gain a lot of experience in such things.
Even without the need for spirit sense, Lex could tell, based solely on its lush plumage and the way that energy flowed around it, how tough and durable it was. It did not move much, as it tried to remain still and blend into the darkness, but its massive eyes revealed its inner shrewdness.
No, the minor size difference would not deter it from being a good mount. Taming it, though, might be a small issue. Long ago, Lex had learned a taming technique. It was, in fact, the one he used on Fenrir.