Most beginning photographers have a misunderstanding that 3-point lighting is the ideal solution. It is true that 3 lights can provide sufficient control to illuminate a portrait subject, but 3 lights will not provide background illumination.

Background illumination principles:

Move the subject away from the background. If your subject is too close to the background the key and fill light will lay objectionable shadows on the background. Attempting to use background lights to correct for these shadows will fail. Keep your subject at least 5 feet or more from the background. A 7-foot subject to background distance is best as it allows for background illumination control and also allows the background to be out of focus.

Studio Space Requirement

This 7-foot subject to background distance requirement is a problem for a small studio space.

There are 2 important factors that determine the minimal depth of a working portrait studio:

Requirement of subject to background distance
Camera to subject distance.

The camera lens focal length for portrait photography is typically slightly telephoto. Most professionals prefer a focal length of 75mm to 100mm (reference 35mm sensor) for portrait photography. A slight telephoto focal length is preferred because it renders accurate subject proportions. On the other hand, a short focal length or wide angle lens will distort the subject proportions and make them look heavier.

When working with a 75mm to 100mm lens and photographing head to toe, you will be at least 12 feet from the subject, maybe more.

What is the minimal depth of a portrait studio to allow for head to toe portraits?
24 feet. You might conclude that 7 plus 12 is 19 feet, but if your studio depth is 19 feet, then the photographers back is against the wall and can not move. 24 foot depth allows for reasonable subject to background distance, camera to subject distance and freedom of movement for the photographer.