You can set this up as a proportion, setting the scale ratio equal to the actual ratio. The scale ratio is .5 in / 1 ft. The actual ratio is x in / 19.5 ft, where x represents the actual length of the model (in inches). For a proportion, we set the ratios equal to each other, giving .5 in / 1 ft = x in / 19.5 ft. The units cancel to give .5 / 1 = x / 19.5. Now just solve for x.