You should be fighting your enemy more or less non-stop from the moment you get to Colonial Age and onwards anyway. That will give you a definite feel for how well he's doing, what size of army he's got, and how hard to commit to any given battle.
Now, from a mathetmatical viewpoint, a larger mass of units always has increasingly better odds. Consider this: A hussar that faces one pikeman, and then another will kill the first pikeman, and badly injure the other. If he faces two pikemen at once, he'll badly injure one, but not kill either. So clearly, it's much better to send both pikemen to fight the hussar at once. The same is true with full armies, only on a much larger scale.
If you're going to commit to a big fight, commit everything you have. But also be prepared to pull out if you sense a loss; you'll need to leave the units the enemy is targeting behind, as units that are being injured move slower, and will slow the entire group down. Also, always keep making units while you're fighting. That's your 'reserve.' There's no need to stop making units while fighting except if you're too lazy to do so.