Building Small Cabin Floor
For the small cabin structure based on the wooden frame, use 2x8" (or at least 2x6") joists from pressure treated timber.
Note:
Some newer pressure-treated wood chemicals may be corrosive to steel connectors, brackets, anchors and fasteners. So manufacturers of those metal parts developed newer more corrosion resistant versions of their product. Some of them are marketed as: ZMAX (heavier zinc coating), HDG (Hot Dip Galvanized) or STT300 (stainless steel Type 316L)
For example, when using Simpson Strong-Tie products, look for the part number ending with "Z", i.e. LUS24 vs. LUS24Z.
The outside perimeter dimensions of the small cabin floor frame needs to be exact size of your choosing (for example, 8x12 ft). It means you need to cut joists to their proper length before nailing floor frame together (for example, 8 ft less 2*1.5 = 7' 9").
Construct floor frame of the small cabin, by nailing joists together with Joist Hnagers.
After small cabin floor frame is built, place subfloor sheeting. Use 5/8" pressure treated plywood. Three of these 4x8 ft sheets nicely cover 8x12 ft cabin floor.Nail or screw plywood sheets no more than 8" apart on all floor joists.
I would stress again that you need to use pressure treated timber for the cabin floor frame and subfloor sheeting. (And obviously support beams if you decided to go with this type of foundation/flooring structure).