That is right, there is a limit on how much you can write in a PM.
Now if replying to a PM, you can delete what the other person has written to you, before you click send. This will free up some of that allowance that is given.
The sent folder will only save sent Pm's if you have set it up to do that.
In your CP, go to Edit Options
, scroll down to Messaging & Notification
, then under Private Messaging
, make sure you have the box for 'Save a copy of sent messages in my Sent Items folder by default
Once that is checked, all sent Pm's will be in the folder.
If you needed to keep any sent Pm's and didn't want it in your folder, you can download them to your computer, with the action from the bottom of your Private Message list, a drop down box shows you what you can do.
I hope that helps