Yes—a speedlight can be used in a softbox, including for full-body portraits, but the practical limit is flash power and working distance. A larger softbox gives softer light, but it also spreads and loses more light, so your flash may need to fire at or near full power, especially if the light is placed farther from the subject.

A 60x60 cm softbox is usually the safer, more efficient choice for a speedlight. An 80x80 cm box can still work, but you may be more constrained by distance, aperture, ISO, and recycle time.

A good way to choose is to test your flash’s beam coverage: fire it at a wall or paper at different zoom settings and distances to see the hotspot size. That helps show how well the speedlight fills the softbox and how much internal depth you need. Since speedlights project a rectangular beam, some softbox shapes and sizes will be illuminated more evenly than others.

So: yes, full-body shots are possible with a speedlight in a softbox, but keep the light reasonably close and expect a smaller softbox to be easier to drive.