I have been made aware of a couple of problems with private messaging feature. One problem is that it sends you an email every time you send someone a message. The other is that as a private messaging thread gets longer, the message’s display gets smaller and looks strange.
I will look into it however I can’t promise any quick solutions. The private messaging capability is a plugin that I added to this forum and changing its behavior would require me to study its current code, modify it and hope nothing breaks since all the code works together. This isn’t easy and it isn’t fast.
For time being if it’s really bothering you just exchange email addresses with other members through private messages and then use email to communicate until the issue is fixed.