you can show at track editing, catch the most outside node. BTB show you coordinates 4 or 5 (i don´t remember exactly). you have a right and left side, also it should be 8 meter or 10 meter. I make always 4m per trafic lane (for bumpy track 1 polygone per meter for flat track 1 polygone per 2 meters). if i want very smal farmers roads, i use track width option (70%). for roads with middle line i make dafault both 4m (road width = 8m).
Use a reference car for checking out the right width, thats the best way and you never fail