If it was me foot print wise I'd say a 50 is better then a 55 as it is wider, which the sen will enjoy more then the little more length a 55 will give. I dislike 55g tanks for most mid size fish its only 12" wide. Yes you will also be lucky if your sen hits 9". So an 18" wide tank would be perfect not so much a 12" as you want about 4 × the length and 1.5 to 2 × the width. You could keep 3 to 4 sens in a 50g with ease. Sorry but my favorite tank size is a 75g you really can do alot and weight wise it's still very managable. I can move them around myself no problems. I'm not little or weak though, I think your avarage person would find it managable also.