Bandil gives a good explanation of the process. I will, with meth, do a fast first growth by chilling the alcohol/acetone solution. I save the first crystals, which I rinse quickly with acetone, I add the acetone to the remaining motherliquor and reduce the motherliquor to saturation again. I add acetone again, and chill. I add these crystals to the first batch. I save the motherliquor remaining and do pretty much what bandil does with it, or let it slowly evaporate over the course of a week at room temperature.
The first batch crystals I get I put in acetone and bring the acetone to near boiling temp. I then add alchol a little at a time until almost all the meth crystals have dissolved. I will quickly filter the entire solution and let it cool down very slowly at room temp in an out of the way location. Given a day, sometimes longer, I get nice flat plates of meth the size of nickles or a little larger. These are very fragile, and I don't try to keep them from breaking apart. I bag the acetone rinsed, dried fragments in sacks and call them diamonds, which they resemble.
The size of the crystal is not an indication of purity. Forming crystals does not make meth any stronger. It may clean some stubborn trash from the mix, which will make the stuff cleaner. Meth is meth, whether its a powder or a transparent or translucent crystal. In so far as recrystallizing tends to exclude impurities, you improve the purity of the stuff, which may have a noticeable impact on its percieved potentcy. You have actually done nothing but improve the purity of the product you do, which in effect is increasing the actual dosage of meth by weight of what you do. The benefit is the exclusion of by products and trash.
Hematite describes a process of dissolving meth in OTC IPA (water and all) and putting the saturated solution in a thermos (preheated) which is allowed to stand a day in a very still, quiet place at room temp before being transferred to the refrigerator, then carefully a day later to the freezer. The crystal size he obtains is amazing. I once asked him where he got that "little bitty quarter" he put in the photo to give a comparison for size visualization. I tried his method, and it does work as advertised. I have trouble being that patient, when the size is less important to me than the cleanliness of the final product.
You can rush it to as little as two to three hours in the freezer with improved product, but the best results are with slow growth over a day or so. One more place that patience pays.