drimades,
You should have some idea of the upper limit. What is the field size for the field in the form's table? Textboxes on forms are usually designed to handle the maximum desired text; if the field is larger than can practically/aesthetically be handled, then usually the control is created with a larger vertical dimension (to display multiple lines of text), with scrollbars enabled so the user can scroll through the text as needed.
Within reason, you can dynamically resize controls at runtime based on their contents. But YOU would have to design the VBA code to size the controls according to your criteria. There is no built-in property that will automatically resize a form control based on the length of its contents.
There is the Can Grow property, which applies to certain types of controls on forms and reports, and which resizes the vertical dimension of a control as needed. But for forms, this only applies when the form is printed, not when viewed onscreen.
HTH,
Ken S.