Does this count civil wars? I mean do you count both sides as belonging to the Roman Empire?

Removing for violating rules on trivia seeking.

After my investigation I found that in second war with Dacians 105-106 AD Emperor Trajan raised about 200 000 solders. Does anyone know another battle?

I'm not sure but I think the Roman-Parthia war was the biggest war. They were the largest nations at the time and most probably had the two most populated nations which would also mean huge armies.