Basic Shed Floor Question

I am going to build a shed 12x16(?). It is 12 feet wide and 16 feet deep and about 8 feet tall. Is 12x16 the correct way to note that?
I have to buy two 16' 6x6 (best HD had). The 12x8 in the Ortho book says use 6x8, but I don't have that. One of my question is since the shed is 16' deep the 2x8 '16 are for the joists that means the floor joists and the skids must match up exactly. Since I haven't built a shed before I don't know, but should the skids be longer than the joist for some sort of overhang? I don't know that benefit that would be. I just didn't think about it being so close. I don't know why.
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