can't see the pic, but 'tiger' usually refers to some gymnotus species of knives. If it is, it would be an adult at 20", though is may get a little bigger depending on the species, you don't see them that big in the trade too often, I've only seen around 6-9", usually for about $25.
I've never kept one personally, but I hear they're pretty aggressive. I'm not about to tell you what kind of tank you're going to 'need' in order to keep the fish. Height really isn't so much of an issue, since they're mostly benthic, but make sure it has enough room to turn around depth-wise and give it a lot of lenght. Personally, I'd give it at least a 6'x2'. But that's just me.