The needle sizes are the reverse of fabrics. The needles get smaller as the numbers increase! Be sure you buy Tapestry needles, like John James or Bohin. If you buy cheaper ones, you risk having burrs cutting or chewing up your floss. DMC makes great floss, but ucky needles.
14 count fabric = 24 needle
16+18=26 needle
20+up=28 needle
These are most common. If you use a 26 gauge needle on 16 count, not much harm, but on 18 count, the same size needle
will make the holes bigger. Whether you wash after that, I don't think they go back to their smaller size.
I am not an ECS employee, just another stitcher.