This problem has been pointed out to Trimble before. It affects all displays with a very large number of pixels-per-inch (as opposed to just large numbers of pixels) - such as Mac Retina displays, 5K displays, and the recently growing number of PC’s with 4k displays. I assume they are working on it, but at the moment there is no announced plan or target for a fix.
In the mean time on a Mac for SketchUp 2015, you can ease the pain somewhat by using the attached collection of double-size images for the inference engine markers. This suite was assembled by John (driven) during a similar discussion over on sketchUcation. To install it, unzip the collection and copy the png files into
/Applications/SketchUp 2015/SketchUp.app/Contents/Resources
inf_retina.zip (22.5 KB)