Changing the default font size for Microsoft Forms forms can be done by following the steps below:

1. Sign in to your Microsoft Forms account at forms.office.com.
2. Open up the desired form.
3. Under the ‘Design’ tab, select ‘Themes’ from the menu.
4. Select ‘Customize’ for the desired theme.
5. From the ‘Font’ dropdown menu in the top left corner, select a desired font size.
6. Save the changes.