Millipedes have two leg pairs per segment, and are harmless herbivores. Centipedes have one leg pair per segment, and are venomous predators.
"Ordinary" is pretty broad. That could be anything from a 1" bug that is more or less harmless to anything larger than other bugs, up to a 12" Scolopendra capable of killing birds, rodents, bats, frogs, etc. Although even in the latter case, you are looking at a venom that is mostly painful (possibly activity penalties) rather than lethal to humans.
What do you want it to be? Probably this is a tiny form useful mainly for stealth. The relevant stats in that case basically come down to movement rate and stalking bonus, maybe some notes about their perceptive abilities.