Hello James,
It depends on the number AND size of the plants you plan on growing.
In a single 55 gal. you may find Nepenthes very cramped after a year
or two. If you are growing Sundews (Drosera), one of the largest
of them, D.capensis, gets to around six inches wide so you can see
that a smaller tank would do good... Keep in mind you'll need either
several inches of soil or some other type of set up at the bottom
of the tank, so room will get used there too. I don't really recommend
Sarracenia for terraruims but S.psittacina, S.purpurea and their
hybrid make for good terrarium plants. Each of them can get to
over a foot round, but that can take quite awhile.
Dave Evans