I ran into that a while ago. Tried a few things like computing how much space I had on screen so I could dynamically resize my forms. However, the thing that would have let me resize text boxes by recomputing how much space I would need for a given text string at a given font size turns out not to work in form context, only in report context.
The U.S. government won't let me run freeware because they have this rule about unsupported code, so the freeware solutions that let me do this in form context are out of the question. I could just take a guess at it, but when you try to put fractional font sizes in a text box, it doesn't always look quite right because not all fractions are acceptable to Access. Thererfore, some aspect ratios and fill factors don't come out quite right either.
I solved the problem another way. I ignore it. This means, of course, that I have some unused space on the right-hand side of my screens. What I tell my users is that I have conveniently left them some working space to "pin" another window while they are working with my application. After all, I am trying to be considerate of my clientele.