When road markings are installed by a highway authority their dimensions are governed by statute - in short if a highway authority instructs the installation of a road marking that marking should comply with the relevant diagram published by the Department For Transport within its Traffic Signs Regulations and General Directions (TSRGD) publication and or Chapter 5 of the Traffic Signs Manual.
To view a copy of TSRGD, including diagrams of road markings, their dimensions and tolerance Click here...
For details of requirements for road marking layouts as shown in Chapter 5 of the Traffic Signs Manual Click here...