I usually go around with a full 20 unit army because thats what all my enemies do...twice. I sent a 10 unit force into Massilia;2 hastati, 2 principes, 1 triarii, general, equites romani, pedites extraodinarii and a couple of local mercs and as soon as they walk in they are met by an aedui full stack, reinforced by the 6 or 7 units they had beseiging my small force in Massilia. On clicking start battle i found that they had 5 units of Gesaetae, 6 of Bataroas and a good few line spearmen.
After this, I now use:

general
equites romani
equites extraordinarii
4 Hastati
4 Principes
2 Triarii
2 Pedites Extraordinarii
2 Velites
and the others local mercs
toatilling 20 units...

I am now beseiged in a gallic town I sacked to resupply by a 20 unit army that immediately beseiged me after i resisted the 20 unit army with a full 4 units of gaesatae by slaughtering them all. After the first battle i lost 500...I am now worried.
Oh and the Sweboz send a full stack down throughg the mountains every few turns to attack Patavium.
And its stil 214 b.c.