Tobacco roots will spread two feet or more from the stalk if there is sufficient, unoccupied soil. It seems to be quite sensitive to soil limitation and competition.
If one transplant in a stand of transplants fails, and is not promptly replaced, the viable plants that surround the opening are often visibly larger and more rank.
Potted tobacco will usually spread its primary roots (there is no taproot) to the margins of the pot, and fill the entirety of the soil in the pot with finer roots.
Field-planted tobacco will grow to a typical size when planted with typical spacing and typical fertilizer/water. Alterations of any of these will directly affect the growth of the plants.
Many home growers are focused on growing huge tobacco plants with huge leaves. While this may maximize productivity (pounds of leaf), it may result in inferior quality, or a quality that is not in keeping with the variety. This latter outcome is common with Turkish varieties grown in the "American" fashion--wide spacing, heavily fertilized soil, ample watering. It all depends on what your goals happen to be. For cigarette smokers, very large Turkish plants may produce satisfactory "Turkish" tobacco. Pipe smokers will likely be disappointed with the same tobacco, since their expectations may be for the more delicate, more aromatic Turkish leaf typical of imported Turkish tobaccos.
So, for American varieties, I aim for a generous 3.75 sq. ft. per plant. With many of the smaller Turkish varieties, I am leaning toward a mere 1 sq. ft. per plant. I've grown Little Dutch at 3.75 sq. ft. per plant, and also at 2.5 sq. ft. per plant, and noticed little difference in the quantity and quality of the leaf.
My pot-grown Shirazi (in a very large, ~8+ gallon pot) grew to about 2/3 the size of Shirazi that was field-planted at 2.5 sq. ft. per plant.
Bob