First off, i must ask, what level are you playing on? Second, my stacks usually consist of 10-15 units in the early game, 20-25 units in the medieval period, and end game i usually have a stack of 30 to sometimes 60 in the end game. You have to break a bad habit ( that i still have trouble with). When you seem worried, do what i do. Save right be4 a DOW and just think "Screw it" and click! Works like a charm
But that also means having enough "kinds" of troops. A massive stack of 30 axes dont go well against a stack 1/3 its size in a city with 100% defense. Siege is king. i have 1/2 of my stack as siege and i can cut my stack size by quite alot of units by using massive amounts of siege, its quite overpowered really.