I have once coded a solution to set the width and height of a textbox programmatically based on the characters supplied. I think this was a school assignment a long time ago.
This can be done with both VBA in a code behind or probably with a macro even. Or with VB.net.
I don't have the code I used way back when, but basically determine the maximum width you are able to provide in character width, the preferred width. Determine the pixel requirement per character for that width. This becomes the textbox width.
Divide your total string character count by your preferred width character count. Round up, calculate the pixel height per character. And use this value times the rounded result for the textbox height. Dirty but it should work.
Any chance you can change to a label and set AutoSize to True? You can fix the width and let the height auto adjust. This should be done before converting to a pdf. In fact all of the sizing should be resovled before pdf conversion.
Another Down voter without a comment, should not be allowed.