Mat size is critical...

People entering a building don't often stop to wipe their feet so it's important that an entrance mat is deep enough to ensure there are enough contacts between the entry matting and the soles of shoes to minimise the amount of dirt carried into the building.

The Australian Building Code's recommendation is that a mat should have a minimum dimension of 1.8m from front to back which allows for 2 contacts from each foot. However studies* have proven that fitting an entrance with 6 metres of an effective entrance matting system can stop 94% of all walked in dirt and moisture. *TUV Rheinland.