The font in which signatures and texts appear is a standard font and cannot be changed. You do have influence on how large the texts eventually end up in your signed document.
The larger you make a (signature) field, the larger the final text will be. The length of the text also influences its size. For example, if there are two signers, one of which has a short name and the other a long name, and you add two equally sized signature fields, the short name will appear larger in the document than the long name