shed on skids questions

I am building a 12x16 shed on skids, and had a question about the spacing of skids. I have leveled the ground (clay and rock), and will have 4"x8"x16" solid cinder blocks partially buried, for the skids to sit on (4 to 6 each side) to keep them out of direct ground contact. I plan to use 2 (two) 16' skids, either 4x6 or 6x6 treated wood, with 12' 2x6 treated joists, across the top of the skids, spaced 16" on center. A plywood floor will be on top of the joists. I inspected some prefab sheds made by the Leonard company which use a similar arrangement. The Leonard sheds have the skids 6' apart, but I think 10' apart would be more stable.
1. How far apart should the skids be? 6', 8', 10', or does it matter? 2. If I use a 10' span, would I need to use 2x8 joists, or 2x6 joists more closely spaced for example 12" on center? 3. Are two skids a good idea, or should I use 3 skids and if so why? and how should they be spaced? Which would be easier to keep level, 2 or 3 skids?
Thanks for your assistance.
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These are moved around by rollback tow trucks so maximum width would be whatever the max is that one of these tow trucks is wide...Thats a big shed your building are any of the commercial ones that big?
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Do you plan on moving this? If not, there are better ways to go about building it.
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