Use Metric units
Hope all are doing well, While designing a part on OnShape, make sure you are using the units as Millimeter.
Steps for changing the unit:
1. Go to My account -> Preferences -> length units -> Millimeter -> save.
2. Back to the Onshape document -> Document settings (left top corner) -> Workspace units ->set default length unit as Millimeter.
Follow the above two steps, this will permanently changes the units to millimeters.