In short, the plant's roots and rhizome need to be cool at all times and have good gas exchange, which means a loose compost, such as live sphagnum and pumice. My long term successes always involved Darlingtonia being grown in unglazed terra cotta pots -- especially those intended for use with orchids, with the enlarged drainage "slots" -- in shallow trays of cool RO or distilled water. Cool nights are also almost a necessity . . .
Darlingtonia californica grows very long stolons or runners (many feet long). Larger pots allow more room for stolon growth. Whether or not crowding of the roots and stolons effects the growth I don't know. In my experience Darlingtonia seem to grow better in over-sized pots or larger containers.
For single adult plants I'll use a half tray, undrained