sketchup was originally designed for construction uses and therefore only supported 1/16 inch as the smallest unit
*shudder*
Just going to fire a slightly off-topic volley over from the sensible side of the world with the gentle and respectful invitation that either inches or mm are welcome in my life subsequent to any proponent of one finishing recital of an exact approximation of the other:
1 inch = 25.4 mm
1 mm = 0.039370078740157480314960629921259842519685039370079......

That said, STL files just store straight numbers (they don't even have a concept of units), so you get what you get, and might just have to work it out each time...
SD3. Mk2b + glass, heated enclosure, GT2 belts, direct drive y shaft, linear bearings, bowden-feed E3D v5 w/ 0.9° stepper
Smoothieboard via Octoprint on RPi