A basic feature of almost every image editor is the ability to zoom in. That doesn't seem to be possible in Greenshot.
Greenshot 1.3 will add a zoom feature to the editor window. We’re not yet ready for a final release, but you might want to check out the unstable version. You can get it here: https://getgreenshot.org/version-history/
Please note: Unstable versions are not thoroughly tested and might include faulty behavior or experimental features. If you do not like surprises you should rather download the latest stable version.