I don't actually think about deck size as a rule, more as a necessity.
Basically, I find what I want to build around, be it a strategy or a single card. Then, I check to see what accomplishes what I want to build around. Then, I go over it in thinking still, figuring out if it's good enough to justify playing a deck around that. If it is, I think about what hoses me down, and go look at cards that complement the strategy while helping with hosers. If there are too many hosers, I narrow it down to what is most played, and if there are still too many that I can't tackle from a few single angles, I scratch the idea. Otherwise, I go for it.
Then, I do the same process with the other side of the deck, only slightly different. If I'm building around a card, like in a combo, and that will almost surely win me any game I assemble, I'll sacrifice the other side to accomplish it, thinning down the deck as much as possible on the winning side, until it contains only the most absolutely needed cards, and then build the other side to just burn through my deck as fast as possible (if this happens to be the Free Peoples side, I also take care that it can survive long enough while cycling, considering single-moves only). If it's not a combo deck, I think about what is the biggest hosing strategy to what I built. Then, as most players tend to build decks with the single-minded philsophy of "whatever pairs good with my other side", I try to anticipate the most common build that will pair up with what'll hose me, and simply build my other side to hose that, regardless of wether it works perfectly with my first side or if it's a bit clunky. I don't do it if it's completely counterintuitive, though; I still build the hosing side, but try to adapt it, even if it means losing in power level, because when you build a hosing side, perfect power level is just win-more, and reasonable power level with enough hosing still means you get enough tool to beat them.
After this is done, I skim over the deck to remove the chaff, what's cute, what's win-more, what won't ever come up with a deck that size, etc. This narrows it down to a reasonable size most of the time, and even when it doesn't, I find that the deck still plays well enough, if well built. The rest is changed after playing with the deck a few times, and/or to adapt to metagame calls.