3:2:1 (melee:ranged:siege) sounds about right. Mounted units can sometimes substitute for 1 of the 3 melee units.
An army of 6 is almost "big" in Civ5. But you'll want at least 2-3 spare units trailing behind to fill in gaps. One of my bigger attacks on a large/fractal map, I had a total of 9 units (4 melee, 2 ranged, 1 horse, 2 siege) plus a pair of great generals.
I'd say if you're going past 10-12 units in an army, you're going to find it unwieldy. Leave the scrubs at home, pack your front lines with your most powerful melee, stick your 2 best ranged on the ends, and keep your siege engines screened. There's no room in a modern Civ5 game for weak units.
One tactic to also keep in mind is that sometimes, it's better to refrain from attacking if it will leave your unit isolated and subject to counter-attack. Soften everything up with ranged and let units enter your field of fire instead.