If you think your message may be long you better type it on notepad or something and then paste, so you can see where it is cutted and place the rest in another message.
Don't forget to pick the race who receives the message (Winplan does not remind you like gmail does) and if you don't press send it just stays there.
Of course, the best way to talk with your ally (even with your enemy) is via email, I always look for it at the player's profile and if it's hidden I ask him via winplan message.
from far away planet Argentina
My races are Birds and Colonials.