Currently in EndNote you cannot adjust the size of the icons.
If you would like to see this feature in a future version of EndNote, You may want to submit a request for a product enhancement on our website.
http://community.thomsonreuters.com/t5/EndNote-Product-Suggestions/bd-p/en-suggest
You can try the adjusting the resolution of the monitor or using the a feature called Magnifier on Windows to zoom a section to of the screen to be larger.