This shell script walks you through the steps required to change the monitor underscan or overscan value on OS-X, when the Display Preferences are not providing the underscan/overscan slider for a given display. This process was documented by Ishan Sharma here: http://ishan.co/external-monitor-underscan and is a part of BashMatic library.

reduce-underscan.sh

#!/usr/bin/env bash
#
# Script to change underscan/overscan value of a monitor on OS-X.
# © 2016-2019 Konstantin Gredeskoul, distributed under the MIT License.
#
#———————————————————————————————————————————————————————————————————————————————————————————
# EXCEPT AS REPRESENTED IN THIS AGREEMENT, ALL WORK PRODUCT BY DEVELOPER IS PROVIDED "AS IS".
# OTHER THAN AS PROVIDED IN THIS AGREEMENT, DEVELOPER MAKES NO OTHER WARRANTIES, EXPRESS OR
# IMPLIED, AND HEREBY DISCLAIMS ALL IMPLIED WARRANTIES, INCLUDING ANY WARRANTY OF
# MERCHANTABILITY AND WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE.
#———————————————————————————————————————————————————————————————————————————————————————————
#
# How to run this script?
#   1. Open Terminal application (/Applications/Utilities/Terminal.app)
#
#   2. Decide if you want to reduce (negative) or increase (positive) screen size. 
#      You should think of a number of percentage points to change it by, for example
#      it could be -5 (for underscan) or 5 for overscan.
#
#   3. Set the following variable to this number by pasting the following, and changing the value:
#
#        export SCAN_CHANGE_PERCENT=5 # overscan, change 5 to -5 for underscan
#
#   3. Copy and paste the following commands:
#       
#         curl -fsSL http://bit.ly/change-underscan-osx > ./change-underscan
#         chmod 755 ./change-underscan
#         ./change-underscan ${SCAN_CHANGE_PERCENT}
#
#   4. You can also run it as:
#
#         ./change-underscan 5 
#
#   5. Follow the prompts and perform actions asked by the script.
#
#   6. Enjoy being able to see Mac OSX top menu and your dock :)

curl -fsSL http://bit.ly/bashmatic-bootstrap | /usr/bin/env bash
if [[ -d ~/.bashmatic ]]; then
  source ~/.bashmatic/init.sh
  change-underscan "$1"
else
  echo "The script $0 was installed correctly, however underlying library"
  echo "was not. Please install it by running the following:"
  echo
  echo "   cd ~/"
  echo "   curl -so bashmatic.zip https://codeload.github.com/kigster/bashmatic/zip/master"
  echo "   unzip bashmatic.zip && mv bashmatic-master ~/.bashmatic"
  echo
  echo "After that, re-run this script."
fi

I have the exact same issue as @itsazzad