In LC Designer ES2 (version 9) you don't have control over the default font. However in the latest version ADEP Document Services Designer (version 10) you can set the default font.
If you are using an earlier version then you can still have some control.
Set up the fields as you want them to appear, eg font (caption and value), alignment, border, fill etc. Then drag these fields to the Custom Object Library. In new forms you can drag your custom fields onto the pages.
Myriad Pro is a very good font to use though on forms, taking legibility and file size into account.
Hope that helps,