Speaker wire is measured in metric system defined by AWG (American Wire Gauge). This system defines current carrying capacity based on the thickness of the cable diameter -- higher AWG numbers translate to a smaller diameter and thinner cable, while lower AWG numbers translate to large diameters and thicker cables.
When choosing speaker wires you need to consider your personal use case and how far of a run the wire will be asked to make.
• Less than 10 foot runs : 16 gauge wire
• Less than 18 foot runs : 14 gauge wire
• Less than 30 foot runs : 12 gauge wire
If you will be routing the speaker wire through an interior wall or ceiling you'll need to also make sure that the speaker is at least CL2 rated. This rating is safe for in-wall applications because it is coated in materials that are flame retardant and non-corrosive.