Choosing a focal length
Focal lengths are available from 28 mm and up. Choosing a focal length for a tilt lens is no different
from choosing a focal length for a regular lens: The right focal length depends on what you want to do with it.
Some general guidelines: Wide angle tilt lenses are more sensitive to small changes in the tilt of the lens.
Therefore they can produce very dramatic tilted focal plane effects.
However because of their sensitivity, they are more difficult to control.
Short telephoto tilt lenses are better suited for portrait work and for producing the classic "miniature effect."
For general use, I recommend starting with a tilt lens with a near "normal" focal length:
35 mm for crop-sensor DSLRs and 50 mm for full-frame cameras.