So, what is the typical wisdom when it comes to chosing a reel size.  Do you tend to go smaller or larger?

For example, if I'm fishing 6lb line diameter (yo zuri ultra soft #6) is it better to fish a 4, 6, 8 (1500) or a 6, 8 10 (2500)?

It's not the line, it's the rod (I fish #4 on a G. Loomis PR844S). If the rod feels better with a smaller reel, that's the answer. Specifically, I fish 2500 series for the most part, but on a St. Croix Avid AS66MLF I fish a 1000 series.

Smaller reels have smaller spools, I hate line coiling like if were a darn spring, bigger spools = less coiling, fill 1/2 of the line capacity with EL Cheapo line and the remaining 50% with thinner line ( the line you actually fish with ). Also, smaller reels = smaller drag washers.

That 's the reason why I prefer JDM versions with shallow spools.  

Given a choice, I'll generally go with the larger because of the reasons Raul mentioned. The bigger the spool, the fewer the problems, for a given line. The larger spools also cast longer and smoother because for a given distance, fewer revolutions of the line are needed.