I consider anything 90-100 and over as large. Anything over 500 as insane. I might consider 40-75 as medium. With having a 90 they are unbelievably large for a 4' tank. I like my 90 more than my 100. My 150 just doesn't seem like a large tank to me. I think it is because the 90 is so much bulkier and the 150 is more streamlined.