It partly depends on how big you work and how many you need to turn out in a relatively short period of time. About the only downside to buying a large kiln that I can see is that it costs more to run it--NOT because it necessarily draws more power, but because the urge to make larger pieces and fill the bloody thing up for every firing tends to use up a LOT more glass. Bigger molds, more molds--it kinda supersizes your glass budget.
But I'm absolutely for getting the biggest possible kiln; roughly three months after I got it I was thinking of my Skutt bathtub kiln as "that little kiln." Mine is 13 inches deep (and you can talk Skutt into going 15 inches) and it does a pretty good job with casting.
The biggest limiting factor I've found with casting and/or pate de verre is depth of kiln, not interior width or length. By the time you add the height of the mold material, the height of a sandbed/box (if you need one), staying far enough away from the top elements (unless you can turn them off), what you're left with for actual glass height isn't much in a standard 11-inch deep kiln. You wind up redesigning pieces to either be assembled post-fire, or you cast the glass on its side (lying down) and come up with innovative multiple delivery channel methods to get the glass to all parts of the mold.
OTOH, I've also vowed that my next kiln(s) won't be "bendover" models but instead will let me layup work in the kiln at least at sitting height. And I do enough flatwork placing work on the bottom of a deep kiln would be a pain. So...I'm shopping around for a used ceramics kiln just for larger-scale casting. When I find one cheap enough to do some modifications on (controller, extra element on the bottom of the kiln, etc.), I'll probably get it, and keep the bathtub for flat fusing.