is it a common thing that the plugin toolbar of Fredo’s plugins (such as Round Corner) is as small as in this screenshot? Might be related to Retina display but I would be surprised if I’m the first person to discover this…
This is an example of SketchUp’s limited support of high-dpi displays and has been known for some time. Trimble made a few small steps toward better support in 2016, but there is still a long way to go. Fredo added a setting for Retina in his options that enlarges the size of some of the graphics, but the text is still to small to read. There is no other workaround except scaling up the entire display (which kind of defeats the point of having Retina!).