It depends on the fabric and project, but here are some common best staple size for upholstery:

1/4 inch:

This is a good all-purpose size for medium to heavyweight fabrics. It provides adequate holding power while not being too cumbersome.

3/8 inch:

Use this size for extra heavy fabrics or larger cushions or chair seats where you want maximum holding power. The longer legs of 3/8 inch staples grip better into the wood or frame.

1/2 inch:

Reserve this largest size staple for very heavy-duty fabrics like leather upholstery or projects that will endure a lot of wear and tear. The extra length and width of 1/2 inch staples can secure these thick fabrics well.

When choosing a staple size, consider:

The fabric weight

Heavier, thicker fabrics require larger staples.

The part of the furniture

Areas that get the most stress like cushions and seat bottoms may need larger staples.

Durability needs

Long-lasting projects for high traffic areas benefit from larger staples.

I’d recommend starting with 1/4 inch staples for most medium-weight fabric projects. Only upgrade to larger staple sizes if the 1/4 inch staples are not securing the fabric firmly enough. Larger staples can be overkill and cause unwanted dimpling in the fabric.