Andy,
The message clearly states
" You have won more than 200 wars You cannot attack countries at war level 3 Please attack higher war level.
I'm not sure what the purpose of limiting your ability to fight war level 3 countries are based on. Why the size of your empire would matter. It is extremely time consuming and difficult to manage large empires already. The advantage of supplying your countries through direct transfers per month is already limited by a preset limit that can be increased by a booster. 5 counties can supply way more than this would allow. If your in a federation and you have not signed a war treaty then you can not help your fed mates, who more than likely are depending on you. I'm not even sure if you did sign the war treaty if you would be allowed to fight. (clarify this when you can please). The last point, and I could give many more simply it allows people to move near your empire and build large armies leaving you with sneak attacks as your only option, which is really just a one time option.
Thanks for your consideration.
Rob