Hello Markw,
We apologize for the inconvenience caused, as per the description above, the icons in Adobe Reader is large, Is that correct?
Please navigate to Adobe Reader's Preferences from Edit>Preferences(Windows)/Adobe Reader>Preferences(Mac)> General>Select "No Scaling" in Scale for screen resolution(restart required)>Click OK.
Close the Adobe Reader and reboot the machine and see if this brings any difference.
Make sure that you have the latest version of Adobe Reader from help>check for updates, reboot the machine after installing any updates.
If you still experience any issue, please repair the installation files of Adobe Reader from help>repair installation(only for Windows), reboot the machine after repairing Adobe Reader.
What is the dot version of Adobe Reader installed, to identify refer to Identify the product and its version for Acrobat and Reader DC
Is it a Mac or Windows machine and what is the version of OS installed?
Let us know how it goes and share your findings.
Regards,
Anand Sri.