Battle size is set in the options menu. If you're getting low frame rate, turn it down, and it will limit the number of troops on the field at any given time. It has nothing to do with the size of your army ... if your army numbered 10,000, and the battle size setting said 30, there would still never be more than 30 on the field at a time.
OnR is generally light on poly count compared to most mods, but we have not implemented the LOD's yet, so large battles could still be an issue.
That said, other graphics options will probably be more critical ... try turning off 3D grass, if your frame rate is low. Or lower the screen resolution ... at 640x480, M&B will run a high frame rate on the worst piece of junk computer there is. You probably won't have to go that low, but it's an option.