Harold,
Not sure you can make the box itself dynamic to allow for expansion, however you can make the contents wrap and expand by using the 'Allow Multiple Lines' in the field properties for the box you want too expand. You may want to try the Expand to fit on the layout page as this points to an anchor point and the starting location.
As for allowing the form to be fillable in reader, if you have Acrobat Pro, you can:
From the File menu
Select ‘Save as Other…’
Select ‘Reader Extended PDF’
Select ‘Enable More Tools (Including form fill-in and save)…’
Then save the form again. Prior to using the form, the user will be required to enable Java, but they have to do this to even see the form.