Hard to say what would be my favorite, because every size has a certain touch about it. Favorite in game style is definitely the 1mt+, just because of the ease of being able to have recreation on board, and giving you a minimum micromanagement armada. That is game efficiency decision though.
Favorite in style I would probably chose 60k ships, or 62.5k, so that you can have 40% engine dedication exactly. This size seems to feel just right for me when I imagine the ship in comparison to all the usual 7-16k ships around, however it offers little in matters of game strategy, because the big ship advantages (shield, armor, best senors, and potential to multi-role without much main mission sacrifice) only show at even higher sizes, and you certainly lost the small ship ones already too (stealth and divisibility).
I am just about to test formations of these in my next game though.