In AR only whole numbers can be entered into any editable number field, not just for tool widths.
However though, apparent line width can vary depending on your painting’s settings.
Example:
Painting A; 1000x1000 px @72ppi = a print size of 35.28cm
Painting B; 4167x4167px @300ppi = a print size of 35.28cm
A single 500% line made with the Pastel tool in painting A will fill nearly 1/3+/- of the canvas. But a 500% Pastel line on painting B will only fill around 1/12+/- of the canvas.
Likewise at the other end of the scale in the examples above, in painting A there is a noticeable difference between a 1% line and a 2% line. While in painting B the difference between a 1% and a 2% line is practically imperceptible.