Measure the footprint, not the roof
Product titles usually quote the roof size. What matters for your yard is the post footprint plus clearance. Measure the flat, usable area you have, then subtract at least two feet on every side you need to walk around. Compare that number to the dimensions listed on the product page.
Check the surface
Most structures want a level surface. Concrete, pavers and compacted gravel are all fine. Soft lawn on a slope is where people run into trouble. If your yard falls away noticeably across the footprint, you will need to level or build a base first.
Clearance overhead and to the side
Look up before you order. Overhanging branches, gutter lines, service cables and neighbouring windows all shape what will actually fit. For wall mounted and lean-to structures, check that your wall can take the fixings.
Permits
This depends on your city and on the structure. As a rough guide, freestanding structures under a certain footprint are often exempt, while anything anchored to a foundation, attached to the house, or above a certain size frequently is not. Saunas may also involve electrical inspection.
We cannot tell you the rule for your address. Your local building department can, usually in one phone call, and it is worth making that call before a pallet arrives rather than after.
Still not sure
Send us your measurements and what you are trying to build and we will tell you what fits.