As a rule of thumb, always recruit if you're at 'Untapped' no matter how many men you have. As for your standing army, you want at least 100k men. Below that and you're an easy target. Above that and you're a tough target unless they have support. At 200k you're hard to touch even with support, unless you get caught in a two front war. At 300k you're pretty safe regardless, though suicide attackers can still take you down in an alliance war.