LiveAquaria.com is a good site to get simple facts on differant fishes. Most lions get around football size but, as they're not increadibly active animals, they don't require as much space as others. There's a minimum tank size listed on each species. I always believe that more space=happier fish.
With lions you'll need to be cautious when choosing tank mates, however, you most certainly can have more than just one lion in a suitable size tank.
Triggers, some wrasses, rabbits, and puffers are all suitable tank mates, but keep in mind they're going to get big as well. Something to keep in mind when choosing which lion you would like is its size. It's much easier and affordable to find and buy small triggers (especially) than it is to get full grown specimins.
Something I recently learned (the hard way) and posted in another thread, is to make sure the fish you're buying is fed frozen food. For the past couple months I've been working to transition my lion to eat frozen and there's finally an end in sight. As goldfish aren't a suitable diet, live food gets quite expendsive quite fast.
Just something to think about, and after all that, I'm no expert on lions. I have done my homework on them, however, and have found nothing out of the ordinary when it comes to care. Just stay away from the spines... I can't stress that enough