Not sure if this matters, but in the modern US Army, a standard infantry line company (light infantry, not mechanized) is about 100-120 soldiers. A regular battalion is 3-4 companies, but one of those is a HQ element, so it will probably be a little smaller. So figure a battalion would be between 375-450 soldiers; 3 battalions in a brigade, plus a company-sized brigade HQ element, so around 1,000-1,100 soldiers, give or take. Regiments are typically brigade-sized elements, but in the modern Army that term is usually only used in cavalry units, or for ceremonial purposes.