According to the search results, the best derma roller size for hair regrowth varies depending on the goal and the condition of the scalp. Here are some key points to consider:

• Doctors recommend using a derma roller with needles between 0.5-1.5 millimeters for hair growth.
• For hair loss, a derma roller 0.5 mm is recommended.
• To stimulate hair growth, a derma roller 1.5 mm is suggested as it can cause punctures that allow topical and hair oil to penetrate through the scalp better.
• A randomized evaluator-blinded study conducted in 2013 found that microneedling therapy along with minoxidil was statistically superior to minoxidil alone in promoting new hair.
• In a 2020 study, participants applied 5% minoxidil twice daily and microneedled once every two weeks with either a 0.6mm or 1.2mm needle length roller for 12 weeks. They found that microneedling with either length was more effective than 5% minoxidil twice daily by itself.
• Its important to sanitize the roller with rubbing alcohol before use and to replace it when necessary, usually after two months or if needles become damaged.
• The derma roller should be suitable for use on the scalp, and it should be handled with care. To use a derma roller for hair regrowth, focus on the scalp and run the roller across it in multiple directions: horizontally, vertically, and diagonally.

Its important to note that there is limited evidence that derma rollers work for hair regrowth, and most of the research is on the effectiveness of microneedling. Therefore, its recommended to consult a doctor or a dermatologist before using a derma roller for hair regrowth.