@Wilson , Youngbear's (whose name wouldn't link for me) advice is spot on concerning matching style of pot for style of tree. And your tree has a nice, feminine feeling, but the species is not too feminine. I think it would pair well with a rounded rectangle with a dark glaze, or unglazed. The dark glaze would give it a feeling of being grounded, which should compliment the open airy feel of the tree.
From a horticultural standpoint, I would think you would want a to put your plant in a transition contain for a year or two. The pot it is in now is fairly narrow and deep, and elm roots tend to be on the thicker and stiffer side. I would look for something a little deeper than 1/3 the current depth and 2-3 inches wider on each side. Then in another couple of years, you can look for something 2/3 to 1/2 as deep as that container and a couple more inches wider.
