The skirting board you use should be proportionate to the size of your room. Bigger rooms can take taller skirting boards and smaller rooms should be fitted with shorter skirting board heights so they stay proportionate within a room.
Relating to the above point, the era of a building can also influence which skirting board size you should buy. Period properties are generally able to take larger skiritng board sizes as they were constructed with larger room sizes and ceiling heights. In contrast todays average ceiling height is 2.4m which is more proportionate with a middling skirting board size between the 121mm/5" and 171mm/7" sizes. Differing again, buildings constructed from 1950's to 1970's often featured very small skirting between the sizes to rebuild Britain after WW2 as cheaply as possible. These would have been 69mm/3" to 96mm/4" skirting heights, however it could be questionable whether you want to imitate the trands of these decades!