A derma roller is a handheld skincare device designed to improve skin texture and appearance by using tiny needles to create micro-injuries. These micro-injuries stimulate the skin's natural healing process, leading to increased collagen and elastin production, which can help reduce the appearance of scars, wrinkles, and large pores. 
Here's a more detailed breakdown:
0.2mm:
Ideal for home use, as it penetrates only superficially into the skin's upper layers (epidermis). 
0.25mm:
Often recommended as an entry-level length, particularly for facial treatments and to enhance product absorption. It's a good choice for those new to microneedling or with lower pain tolerance. 
0.3mm:
Suitable for scalp use and stimulating hair growth by improving blood circulation to the hair follicles. Hair Growth Specialist recommends a scalp roller with 0.3mm needle.