Every wood floating shelf is made to order, so the sizing question is really about what works for your wall and your load rather than what's in stock.
Depth is the first decision. At 6" deep, the shelf has a slim profile suited to display-only applications: framed photos, small plants, candles, books. At 8" you get enough surface to mix display with light storage. Ten inches handles most kitchen and bathroom applications comfortably, including dishes, glassware, and grouped objects. At 12" deep, the shelf is doing serious work: full rows of books, stacked dishes, heavy equipment, or anything with a significant footprint. The floating shelf depth guide covers the full breakdown with room-by-room recommendations.
Length runs from 12" to 72" in any increment. Short shelves in the 12"-24" range work well in tight spaces, above doorways, and as accent pieces in a larger arrangement. Mid-range lengths from 36" to 48" are the most versatile for single-shelf installs. Long wood floating shelves from 60" to 72" make a strong statement on a feature wall or above a piece of furniture that spans most of the wall.
Weight considerations should inform both depth and length. Longer shelves at greater depth under significant load benefit from additional bracket points. The Hovr system is rated at 150 lbs per stud; on a 72" shelf hitting multiple studs, that adds up to serious capacity. For heavy applications, the rule is simple: more studs, more brackets, more confidence.