How to decrease the field length of long text tag? | Adobe Sign

Decreasing the font of the text tag:
The first open curly bracket decides the font type and size of the field. You can decrease the font size of the later part of text tag to decrease the field length. 
For Example, you can change {{Birthday_es_:signer2:isdate(format=dd/mm/yy)}} to

Using Text tag Shortening:
A shortened text tag consists of two parts:

1. Reference Tag: The reference tag controls the size and location of the actual form field in the document. A reference tag has the following syntax: {{$shortened_name}}.
   For example, {{$r}} defines a reference tag where "r" is the short name of the tag.
2. Definition Tag: It refers to the complete definition for a specific tag and can be defined anywhere in the document.
   A definition tag has the following syntax: {{#shortened_name=Standard Tag Syntax}}.
   For example, {{#r=[]ReceiveCommunication_es_:signer1}} defines a complete tag definition for the reference tag named "r."
   To learn more about text tag shortening, see Shortening Text Tags.