I voted for default, since that's all I've ever tried in my campaigns.

I did have a stab at double-size units once, in Shogun days, but found it very off-putting, for all the usual reasons.

In Shogun, my biggest fight wasn't with the enemy factions, it was with my own economy - constantly getting average, poor or disastrous harvests and never having enough money to keep pace with the size of enemy amies, even on default size. To add the higher cost of the larger units just made matters worse.

Variable harvests are a thing of the past, though, so I'm certainly willing to give it a try with the bigger sizes, one of these days.

An additional strain on the economy will be the need to build more unit training centres, to reduce the impact of the 2-year training times and it's good, in a way, that this motivates the player to spend time and trouble building the economy to be able to afford this extra infrastructure, instead of being able to rush a nearby faction into oblivion in the first few turns. Retraining of damaged units may also figure more into the game, as a means of saving time. For me, the logistical problems are a part of the overall challenge.

By the way, I forgot that there are three sizes bigger than default, so I hope I voted right! (Default: RK=20, Cav=40, Inf=60, BI/Pez=100)