One footstep is about 128 units (8 squares). However, if you use this as a unit of measure for scaling the dimensions of a level, everything will come out looking roughly twice as big as intended, and players will look about 3 feet tall.
To create a scale model of an environment, I use the following guidelines ...
For example say, a room in your home ...
Record height / width / length of room in inches.
Divide measurements by 4.5 to give the number of 16-pixel grid squares in UnrealEd.
Round up / down to nearest whole number, to give whole grid squares.
Multiply by 8 to give the number of pixels used for defining a brush.
I created a simple Excel worksheet as a ready-reckoner that performs the necessary conversions ... you just enter the dimensions in inches and it tells you the grid-units and pixels needed to create a brush that "feels right" within UT. The worksheet limits scaling errors to a maximum variance of 4 inches from their actual value, and still ensures that values entered into UnrealEd correspond to complete 16-pixel grid squares. Just email me if you'd like a copy.