Likely during its peak near the 1590s, and under the rule of Suleiman the Magnificent, the Ottoman empire's army reached it's largest size. Although there doesn't seem to be any size numbers online or in books I own, we know that Constantinople was bigger than any European city, possessing over 500,000 inhabitants. I think it would be safe to assume that the Ottoman empire had a competitive size army, numbering in the hundreds of thousands. Although it is very difficult to predict how many causalities this army could have inflicted, their army was very effective in the art of war, and had advanced military technology at the time. They would be at war or at risk of war with the countries that they shared borders with at the time, so many North African nation states to the South, Austria, Italy, Russia and other European nations to the North, and the Persian Empire to the East. They also fought against other European powers for supremacy of the Mediterranean, so France and Spain have fought naval battles against the Ottomans at some point.