There have been many reports of SketchUp issues on high-dpi displays, starting with the original MacBook Pro Retina, and now including the iMac 5k and emerging Windows 4k displays. Mainly these involve items being too small or lines too narrow to see. Trimble is aware of the situation, but has announced no schedule for addressing it.
As a near-term workaround, you can navigate Finder to the SketchUp app (/Applications/SketchUp 2015/SketchUp), right-click and select GetInfo, and then check the “Open In Low Resolution” box. This will make SketchUp run as if the pixel size on your display has been doubled. Everything will be less crisp and the rasterizing “jaggies” on lines will be double-sized, but the issues you have found will be eliminated.