A foot was a unit of distance measurement commonly used in the United Kingdom, particularly during the Third Doctor's exile on Earth.
The Doctor's time sensor was calibrated to read in terms of Venusian feet and miles. Though he once reckoned that there were about .225 Venusian miles for every Earth mile, he didn't give an exact relationship between Venusian and Earth feet, nor between a foot and a mile. However, Jo Grant did note that Venusian feet were much larger than Earth feet. The Doctor confirmed with a pun, noting that Venusians themselves had feet so big they were always tripping over them. (TV: The Time Monster)
By one account, the Dalek War Machines encountered by Ian Chesterton in the Dalek City were but three feet in height. (PROSE: Doctor Who in an Exciting Adventure with the Daleks)