Leena,
There is a way to do this, but it is not advisable.
To get the font to change size so the field can accept more characters after you have filled the field:
- Select the field.
- Click on the XML Source tab at the top of the Design area.
- In the XML of the object find the font and then the font size.
- Change the current font size to "0pt".
- Click back to the Design View.
If you test this you will see the text shrink as you add more and more characters. The downside is that it can become unreadable.
It would be best to set a character limit on the field so that you halt the entry of characters and thus the shrinkage.
From a forms development and forms management perspective allowing unlimited characters in a fixed width field should be avoided. Either allow the field to expand horizontally, vertically. Alternatively, you should identify the max characters required to fulfill the data requirements of that field and design the form to accommodate those max characters.
Regards