Issue with text box scroller in legal size document

Hi, I am trying to set up a signature field in the document in Adobe Sign. The document is pre-signed by the other party, so I need to keep a record of this signature. Adobe perfectly recognized it (I cannot flatten the file in this case to solve the problem I explain further). The size of the document page in PDF is 8.5"x 11" (legal).

When I open it in Adobe Sign to set up fields, I see the big text box with the main text with a scroller (so the full text is not visible on the page, you need to scroll it). The text is preset to Auto and Multiline.

The signature field should fit in that text box, and I managed to add it there (first I placed it under the text box, and then dragged it over the text box). When I send the document for signature, the next signatory sees the text form that text box  in very small font, and the final signed document has it in a very small font, not the same as it looked in Adobe Sign. I tried to preset font size to 8 to make the scroller bar disappear and fit all text in the field, but it didn't help.

The original PDF I received from our partner is perfectly fine, I can see the full text and it's normal size. So I am not sure if the legal size of the document creates a problem and how can I fix it? Thank you.