I can tell you what the show has told us so far.
Renly Baratheon is blowing everyone away with 100k soldiers in his army. The Season 2 premiere told us that the Tyrells have joined him, so he basically has two of the wealthiest kingdoms.
Robb had 20k towards the end of Season 1, and he may have more now, although not nearly as many as Renly.
Stannis has a small army.
The Lannisters have enough that if Robb and Renly joined, the Lannisters would be outnumbered 2:1. So if Robb still has only 20k, they have 60k; if Robb has doubled to 40k, they have 70k, etc.
Right now in army size, it would go Renly > Lannister > Stark > Stannis. However, up to this point Robb has been the best leader and commander of all four, which gives him a bit of a force-multiplier.