How to change the text cursor (caret) size
Your quest on how to change the text cursor is an unusual one outside the norm of utilities offered. 
Therefore an Apple Tech Supporter or Developer  may advise you as to the best avenue. Please go to apple support: https://support.apple.com next click on the white area box: Search Apple Support... a drop down window appears. Select 'Contact Apple Support' now click on 'talk to us' and select the method of contact i.e. personal phone call from Apples Support team or on-line chat.
The Apple Representative will guide you through the process as needed if available.  To the Best of my knowledge, it's best not to guess or take the advise off of the Internet unless trusted. Ask Apple directly sense this is a valid request but not offered in the OS X Sierra utilities.
Authored Statements:  ysureican Fogarty