Go into your text app and tap thr menu in the top right corner then go to settings. In there you should see a setting to turn off or on sending large messages as mms instead of breaking it into multiple SMS
Is there a reason you don't want them sent as mms?
also remember that all current Koodo plans have SMS and MMS unlimited and free. By removing the auto-conversion whenever your SMS is too long the message will break-up randomly into 2 or 3 separate messages. You should keep that in mind mate.