25 Saddlers Harness needles in paper envelope. 

Size 1            55mm length x 1.02mm diameter
Size 2            55mm length x 1.02mm diameter
Size 3            55mm length x 1.02mm diameter
Size 4            48mm length x 0.86mm diameter
Size 18 (1/0)  57mm length x 1.09mm diameter
Size 17 (2/0)  60mm length x 1.42mm diameter
Size 16 (3/0)  62mm length x 1.63mm diameter

So what exactly the difference, if you not sure?

004 (size 4) - the smallest size, suitable for small leather goods including wallets, bags, accessories
001 / 002 /003 (size 1, 2, 3 ) are slightly bigger , but absolutely identical in size, i still have no idea what the difference is - suitable for general leather work, including bags, belts
Size 1/0 is larger again, next size, same aplication
Next up:  size 2/0 and 3/0
Size 3/0 is the largest needle from them all and used mostly for heavy duty application like saddle work.

Hope it helps.