How many mm thick is the string?

The elastic for this product has been carefully chosen for optimal performance for its intended use. All characteristics including its superior quality, thickness, strength, elasticity etc. have been evaluated for this purpose. Using other brand elastic may not produce the desired result. Thank you for your interest in Ez Necklastic.

Is this elastic flat like floss?

its round like regular string, and it is rubbery that if you stretch it, it will not rip.

Can this make necklines smaller, or just prevent stretching? If I want to make a neckline a bit smaller, will there be wrinkles?

Yes, it is used extensively to make necklines smaller & is a primary purpose of the product. It will not bunch (i.e. wrinkle) if you follow the instructions. The key is to make the 2 stitches, as per instructions. This allows the neckline to be pulled in mostly from the back so that the front is very smooth looking. It really works amazingly well. Of course if the neckline is very much over sized the results will not be as good but will still work pretty good.