I'm not sure about adult cupido, but Biotodoma wavrini can closer to 6" and thick!!! I was shocked the first time I saw adults. I deffinately wouldn't keep a pair in a 29 gallon, heck I passed on smaller ones even in a 40 gallon breeder.
The fish in the pic isn't a cupido though, nor wavrini ... it's an undescribed Biotodoma. The way to tell the three about is the location of that spot on it's side. On Cupido it's completely above the upper lateral line. On wavrini it sits below the upper but above the lower lateral line. On the undescribed species, the upper lateral line splits the line (which it appears to be doing in your pic).