I think it's a pain in the ass, that you are not allowed to make brigades smaller to free up recruits or weapons for the armory. I guess it's a design decision, because you would have to implement an experience system for the free recruits. But right now I'm sometimes forced to reload an entire savegame, when I make a bad decision in the army management screen, erasing all other changes I made until then, which can be quite frustrating, when managing bigger armies.
My suggestions: Just make all recruits, that are freed up by downsizing brigades, lose their experience. It's a bit harsh and you would like to keep those situations to a minimum, at least when managing experienced brigades, but at least you could toy around with the numbers of new brigades, that you may or may not want to raise. Or give it a global "Apply" button, making you able to queue up multiple changes in brigades at once, again making you able to toy around with the numbers a bit.
I'm surely a bit on the OCD-side of live, wanting to have evenly distributed even numbers of soldiers in my brigades, but for me the system currently in place is suboptimal and I cannot be the only one feeling like this.