It is my impression that this topic is one of the most asked questions on the DocuSign Community....there are alot of previous posts about Signature size. Lets start by providing the DocuSign article about field resizing:

https://support.docusign.com/guides/ndse-user-guide-adjust-field-size

Now based on the article the Signature Tag defaults to 100% of the size, so unless the tag was sized lower when it was placed, there is no way to go greater than 100%. The article, however, also talks about the document and has several points regarding scanning or document resolution....

• The resolution of your uploaded file
• The page size of your uploaded file

See the very bottom of that same article for the recommendations that the article presents.

I have not ever had to deal with anyone yet to complain about the Signature size so I cannot comment specifically on the topic as to whether I had another approach like education, or even scan resolution. I am actually hoping to have this problem so I can see what steps I can take.