This interactive table of nuclides shows the radius of a bare neutron to be -0.1149 fm (femtometers). How can that radius be negative?
The page uses interactive mouse-clicking, so I've included a screenshot with the neutron info at the bottom. The radius is circled in red.
To get to the neutron info, click on the bottom-most square on the chart.
Also, you can select color-coding with the drop-down menu on the right, and choose "radius (fm)". Note that only the bare neutron has a negative radius, according to this page.