It should be really big. Let´s assume a wels will grow to 1,5 m or perhaps 2 m in warm climate after several years. You should a pond which is at least several times longer than the fish´s length. At 2 m it should be at minimum 6 m or better 8-10 m and a corresponding width of at least 3m. But I think a circular pond is really not such I bad idea, I also already thought about it. Of course not with streaming water like a river, this would need way too much energy and would have no use for a wels, because they don´t need streaming water. But in a circular pond with an island or observation-room with windows in the middle, you could give it the impression of a much structurated habitat, where it can swim longer ways. I have already looked at swimming pools, which are comparably cheap compared to their huge water volume (a pond would not really be much cheper). A round pool of about 6 m diametre with a little island/observation-room would surely look very cool. You would also need only a comparably small area for the pool. Of course there should be decoration in it, sand, stones, roots, water plants etc.