Smaller is not always better. You need to take into account what you're going to use it for.
If you're going to grow in a garage or some other location where space is limited, then maybe a smaller grow tent is best. On the other hand, if you're growing outdoors, or in a more spacious location, then you may want to consider a larger grow tent.
There is no hard and fast size limit on these tents. The size of the tent will depend upon how much space you want to dedicate to growing and what style of grow you want to do.