To be honest, the message size isn't something I'd worry so much about. UNLESS you're writing whole chapters of books, a 200 character message isn't going to break the bank.
What MIGHT break the bank is all the surrounding envelope and tracking metadata. A big part of iMessage traffic isn't the message itself, but the delivery conformations, device status checks and read receipts that are exchanged.
End result: a text message that would take up 160 or so bytes via SMS, can consumer up to 15 kilobytes when wrapped as an iMessage. (Source 1, Source 2). And that's before adding any photos of video.
If iMessage is really eating up your data usage, then it would help to turn off read receipts. If it's still a problem, you might need to just turn it off altogether when not on Wifi. But even though iMessage uses way more data than straight SMS, it's not a whole lot compared too all the other stuff you use your phone for.
Thanks for all the thoughts I was more thinking about pictures getting sent or wanting to send pictures because they can be a few megs at least. I guess not though, no worries. Thanks