Setting text frame size and scale in place note tool
Text frame size appears to change as the height and width of text is adjusted or if text in text frame shape is modified to add or remove characters. What factors determine size of the shape and is there a way to specify an exact size for the text frame.
You cannot set fix size for frames because when text added or deleted, the frame size will be change to cover that text.
http://communities.bentley.com/products/microstation/w/microstation__wiki/microstation-dimensions.aspx