We frequently use this sensor to track lines made from 0.75 inch wide black electrical tape. The sensor should work with some thinner lines, but the contrast of the line with the background, sensor mounting height, the noise in your system, your scanning algorithm, and a lot of other factors could all have an effect on detection.
Do you have any particular thickness in mind? Which QTR-8 sensor (the QTR-8RC or QTR-8A)? Do you already have the sensor? Can you tell me more about your application?