Robb had 18k, but I think that is without Manderleys who joined later. Riverlords if I am not mistaken had around the same, with Freys obviously having the largest army, but Edmure had already called his bannermen, and why every bannermen has significantly less soldiers than Freys, there were still a lot of them. So, I think it is safe to assume that Robb had 30-35k soldiers, with one third of them being under the command of Roose. His army and that of Lannisters were roughly equal on size.
Counting his army this way is a bit counter intuitive since it was split into 3 different commands, the 6k horses he led to relieve Riverrun, the River force (unspecified) under Edmure's command, and the 12k foot under Bolton that already suffered heavy losses at Green Fork and later Duskendale. I may be a bit off here but can quite confidently state that at no point did he field 35k soldiers simultaneously against the Lannisters, which speaks to his military talent that by the time of his death he's never suffered a defeat.
Not sure I agree (remember this). Stannis had under his command most of Reach lords. Of course, Tyrells forces and Tarly stayed loyal to Tyrells, but I thought that a lot of Reach lords joined Stannis. Storm lords alone (or with Florents) couldn't make an army as big as that of Stannis.
No, Stannis's command at Storm's End was 5k of the Dragonstones lords and mercs, he then inherited the majority of the 20k horses Renly led. Loras took away Renly's corpse and reached Bitterbridge before Stannis's messenger could, assuming command of the 60k foot soldiers there. The Florents were the only major Reach house to declare for Stannis, so it's close enough to make no matter anyway. I mean if we get banal about it we can probably peruse TWOIAF and find a vassal house or two of the Florents and add them to the mix.
Tyrells might still have had the largest army, but they kind of fought a lot, and suffered losses both in the battle of Blackwater in addition to the attack from Victarion. On the other hand, Vale army was totally absent from fighting (no dead soldiers, no economical losses, a lengthy war of course costs a lot), and they have a shitload of knights. Of course, Dorne combined with the Black Company forces should be also very strong.
I'm sure they suffered losses, but you seem to overestimate it. They entered the fray in KL when Stannis's forces were already occupied in the crossing, took them by surprise and crushed them. The victor in a rout tend not to suffer a lot of losses, and Victarion's battle were basically raids against small garrisons in 4 tiny islands (the Small Shields). How many can they lose? A thousand, two, top, not likely to even be close.
The Vale is strong for sure, untouched by the fighting and one of the great houses, but the repeated usage of 'Knights of the Vale' seems to have create a false impression. Nowhere in cannon was it stated that they are uniquely strong when it comes to horses, especially when you take in to account their mountainous terrain and the tremendous cost of an armoured knight in the medieval period. If anything, the Lannisters are more likely to be able to field the most mounted units with their financial advantage. Dorne by Doran's own admission is the least populous and poorest of the 7 Kingdoms, so the boastful claim of '50k shields and spears' by Quentyn is doubtful as well.
At least GRRM kept his promise that at least one book will be released this year. Let's hope that he finishes Winds of Winter this year.