Greenshot-RELEASE-126.96.36.199 & Greenshot 1.3.122 - 232e20cb72 (64) bit)
Not a developer, hope I added this correctly and am not annoying you. Thank you for the great work on Greenshot!
To find a solve, open https://github.com/greenshot/greenshot/issues/115 and scroll to post from kindtomato who explains how to find the setting and change the icon size.
For 2160x1440 Resolution I reccomend 32x32