Does the size of the microneedle determine effectiveness?

The needle size is standardized but the depth of penetration of the microneedles can be varied depending on how aggressive of treatment is required.  I typically recommend a series of 4 sessions spaced 4 weeks apart and then maintenance treatment every 6 months.

Thanks for your question, but I can't answer until I know what you are treating.  Microneedling may not be the best treatment plan for you.  Depending on your needs/complaints, lasers or injectables may be the best option.  If you dermatologist doesn't have any energy devices, he won't recommend them...  Best of luck.

Good morning and thank you for your question! Depending on what you are trying to treat it can take a series of three to six microneedling sessions. We recommend the treatments be spaced about four to six weeks apart with maintenance every six months after that if needed. Best of luck! For more information please feel free to visit our website!

To answer your question many factors determine the efficacy of micro needling, Excluding the skill sets of the Specialist (this is the most important factor as one has to know the depth and passes), other factors that come into play include- what your aim is ? Are you treating pores, fine textural changes, or are you targeting deep fibrotic scars? The depth is important, and also energy. Energy based micro needling devices provide controlled heating, this stimulates more collagen. So now energy levels come in to play. How long do we deliver the energy is called the use duration. The longer the PD the longer the energy delivery, and more heat gets into the dermal layers. If energy is used, is the needle insulated or not? Is energy delivers in the target area ( 3 mm vs 0.5 mm for example), the number of passes also count, both horizontally and vertically (shot count), technique, when the skin is stretched, needles penetrate deeper. Infusion of PRP also counts for heal times and also better remodelling and healing of skin. After care . Equally important. Micro needling can be as simple as 'Im using a 0.5 mm needle, or as complex as above. ' As specialist all the above goes through my mind when I am about to perform energy based microneedling. (Skin colour also plays a part, even though companies say its colour blind too much energy will give brown dots on the surface if the treatment is too superficial.) Bottom line- be guided by a dermatologist or plastics who understands and practices this type of microneeedling, this way you will get the best, and safest possible results. Of last note, always go for a treatment to meet your end goals, namely consider lasers, peel, skin care etc..... see someone who can offer the lot, and tailor a treatment for your concerns. Dr Davin Lim. Brisbane . Australia. More info in the web ref. below. 

A lot depends on what you are treating and the goals.  Every 2 weeks seems close together.  I like to space mine 4-6 weeks apart.  The needles cause a micro-pore and damage to the dermis.  This has to be repaired, and part of the repair process is for the body to stimulate new collagen.  I like to give this enough time to occur before further treatments.
Also, not sure what is meant by 0.25 microneedle.  I use SkinPen, which has 14 needles on the tip.  I can adjust these to penetrate anywhere from 0-2.5 mm deep.  The depth is dependent on where and what I am treating, and I will adjust within a single treatment.  For example, I treat acne scars in the cheeks much deeper than under the eyes or on the forehead.
I also like to add PRP with microneedling.  This is also known as the Vampire Face Lift, and it speeds recovery and helps rejuvenate the skin.
Good luck to you,
Dr. Powell