i think you should start with a multiple of 5 cards.
if you don't, some of your initial purchases could be gotten a whole shuffle before an opponents, which is needlessly unfair.
in a two player game, you could require 4 piles, or 12 provinces to extend the game, but I suspect these would hurt the experience.
After all, a good game ends when you are still having fun, and a bad game goes a little past that. if you manage to extend the game in a way that turns it into a big of a slog, you may have a worse time.
It's probably better to load the slog into the opening, like you would with a 15 card deck, because once your deck is fianlly trashed down, it will end like an ordinary game.
You can also slog up a game a bit by designing the kingdom with poor trashing. if it is impossible or laborious to trash the estates and/or the coppers, the game will go longer. For instance, if moneylender were the only trasher, you will still have, at best, 4 dead cards in your deck at the end of the game.
Strong attacks also slow the game. And if you have witch, AND weak trashing, the game can slog. I don't want to play slow boards all the time, but I find them to be fun in moderation.