How do I know if the Free size garment fits me?

If all of your body measurements fit into the sizing chart in between sizes XS (Extra-Small) and M (Medium), then yes, a free (one-size) item will fit you well!
If you usually wear L size, then a free-size garment might fit, but it will most likely be a very tight fit.

"Free size," also known as a "one-size" garment, means that the item is only made in one size.

• Free Size clothing suits best body sizes from Extra-Small to Medium (XS, S, M). Depending on the design, it usually suits sizes Large too, but might be too tight for size XL. Usually, free-sized clothes are too small for XXL and Plus sizes. For more information please always refer to the product photo, description, and lay-flat sizes.
• Free Size Women's clothing suits women's bodies sized from XS to L; men's bodies sized from XS to S.
• Free Size Unisex or Men's clothing suits women's bodies sized from S to XXL; men's bodies sized from XS to XL.
• Free Size accessory fits any body size. Depending on the design and product type, it might be slightly too loose on XS sizes and slightly too tight on Plus sizes.