Sewing machine needles (at least the ones I know about, anyway) the bigger the number, the larger/thicker/stronger the needle. The needle length stays the same for each system.
Pay attention to the type of needle your machine will use.
It is worth reading the articles about machine needles.
Just to make things more interesting/confusing -
For needles for handsewing - the opposite is true. A #12 "between" needle is about 1-1/4 inches long and not much thicker than a hair and a #8 "between" needle is about 1-1/2 inches long and quite a bit thicker than the #12 "between."
The numbers of handsewing needles refer to the thickness of the needle - not the length of it.
I have no idea how the numbering systems came into being.