Thank you for reaching out to us.

A narrow stain is measured in terms of its area, and in this respect can still be compared to the area of a US penny. Sometimes, this is done through estimation.

However, you may not need to estimate in this case. A light brown with no hint of a reddish tint (e.g., the color of coffee with milk or lighter) is not a niddah color. The stain that you describe seems to fit this description, which would mean that it did not invalidate your clean days, regardless of its size.

Additionally, if a woman is reasonably confident (e.g., based on color and position) that a stain can be attributed to fecal matter, she can disregard it with no need to ask a question.

For the future, Yoatzot Halacha who can assess stains and bedikot are available in an increasing number of communities, in Israel and worldwide.

We hope this helps. Please let us know if we can be of further assistance.