I bought a womens seamless cami online but it didn't have a size, how come?

[dandelion:]]

All it said is "One size" so now I don't know if I ordered something too big. I hope it's small since I'm pretty small. What does "One size" mean? What's a seamless cami?
How can it fit all....like if I was size small or extra small would one size fit me?