What length mirconeedle for old indented stretch marks?

This is difficult to answer without pictures. A combination approach gives the most improvement to stretch marks. I recommend a combination of microneedling/prp, lasers (Erbium/Fraxel Laser, Excel V, Pulsed Dye Laser) or radiofrequency devices. Stretch marks can't be cured and it will take a series of treatments lifelong. I recommend you see an expert for a formal evaluation. Best, Dr. Emer

Let me propose a third option.  
For best results use a combination of shorter needles even daily and longer needles monthly.   Stretch marks involve damage not only the top layer of skin or epidermis but also to the middle layer of skin or dermis.  The shorter needles, 0.3mm in length, are used to help the epidermis rebuild and prepare it for deeper treatments.  Ideally the more superficial treatments are done daily at home using a dermal roller.  The deeper treatments, 2.5mm in length, are used to help the dermis build new and reorganize old collagen.
The deeper treatments should be done in a controlled environment for two reasons.  One, the use of topical anesthesia, such as lidocaine, will make the procedure much more tolerable. Two, the procedure should be done in clean or aseptic conditions to reduce the risk of infection.  Please seek out a well-trained and experienced provider to help you.
Finally, microneedling is a great procedure for a number of conditions or for just encouraging the skin to build more collagen.  However, nothing can remove stretch marks the best outcome is minimizing their appearance.
Good luck, Dr. Z.