Listen to Dex, he's the terrarium master around these parts from what i've seen! 55 gallon sounds good to me, im using a 30 gallon at the moment and have fit my modest collection in it comfortably.
Aww come on now, stop that. Plus there are other members that are much more masterful at terrariums and vivariums.
*coughhackswordshackcough*
If anything I am the master of cheap and lazy terrariums.
Anyway, definitely consider what plants you are growing. . . I particularly do not recommend having a lowland terrarium seeing as there are only a few plants that stay small enough for any size terrarium for any amount of time. (N. bellii, N. campanulata).
Almost all other lowlanders will tragically outgrow the setup in no time. . .
As far as lighting it really depends on what kind of lights your using. Like I already said, I recommend T5's, but I have also used just Compact florescents too.
My current terrarium is about a foot and a half deep and the shortest of plants get a good amount of lighting. If I want to blast certain plants with light (N. argentii, Heliamphora, Sarracenia) I have them sitting on upside down pots to raise them up a bit. Some people might try and raise the entire terrarium up so that the ground is a steady height, but if you're growing Nepenthes that will not work. The shortest of plants that are on the bottom (Drosera, Utricularia) and are in the shade may receive a modest amount of light, but any plant that gets within the <6 in mark is going to experience extreme amounts of lighting. Vining Nepenthes will inevitably be burned or have to be redirected downwards.
Ultimately terrariums can only really be transition homes for most Nepenthes. There are very few Nepenthes that you would be able to grow in the long run in a terrarium (unless of course you have an absolutely huge terrarium).
That is why most of my plants are small seedlings, and as they grow older I will inevitably have to trade away or sell the larger plants which are no longer fitting in the tank.
That is also why I am looking for another N. argentii.