Stretch marks extend through the epidermis and reach the deep part of the dermis (called the reticular dermis). The thickness of the skin (epidermis + dermis) is not the same throughout the body. It varies, depending on the location on the body and it also varies individually. The thinnest skin is on the upper eyelids (ca. 0.5 mm), the thickest on the footsoles (ca. 4 mm). For the rest it is about 1.3 mm to 1.8 mm thick (the skin on the back can be 2 mm, especially in men).
Below the skin, there is a thick layer of fat, then muscle and then bone.
A 1.5 mm roller penetrates about 1.3 mm into the skin, which is very deep into the dermis and it is sufficient for stretch marks.
You can use a 2 mm dermaroller but it will be more painful and likely it will have no extra effect comparing to the 1.5 mm size. As a beginner, you should always start with shorter needles and when they get blunt, you can buy longer needles.
Currently, there is no method (including dermarolling) that can remove stretch marks. Currently, stretch marks can only be improved:
https://http://owndoc.com/dermarolling/dermarolling-microneedling-hype-realistic-results/It is certainly realistic to get improvement in 3-5 treatments (with >1 mm size) but it doesn't mean the stretch marks will be gone and it doesn't mean that everybody will get improvement after only 3-5 sessions. The clinics
just recommend 3-5 sessions and depending on the results, you will need more.
We have been always advocating to use the combination of a 1.5 mm roller + the single needles for stretch marks. The single needles penetrate deeper than a dermaroller and they can target the marks densely and individually.
Single needling is laborious but very effective in combination with dermarolling.