I have noticed its better to have the maximum number of brigades first, then increase size. Fun fact AO skill increases the brigades you can have, as well as size. The AI wont scale to your POTENTIAL size, only what you actually bring. so high Organization lets you bring 20 brigade corps, of 2000 men each. and the AI is not scaling to the 2500 theoretical maximum, just what you brought. this lets you fight the AI a little more easily. you can also do a little math at the beginning of the deployment screen if you have 4 recon to help you figure out when the AI hits a scaling point. Look at the number of brigades he is bringing, then divide the brigades into his total soldiers. gives you a rough idea of the size the AI is scaling to you. I did this on my normal union campaign and had much better scaling results then on my easy union campaign.
I cant stress enough though, on the big battles, make sure you can fill the brigade requirement or as close to it as possible.
The small battles tend to have division size elements. so you will have more brigades then you can deploy, just pick the ones you want in the deployment square at the start of the battle, and cycle through your troops to get them all experience over each battle. IE dont fight every battle with the first division of your corps because the other 3 will be green. and spread the love, allow your 2nd/3rd corps to fight small battles too.