MOT rear fog lamp

[jaky cakes]

Anybody know if there is a legal minimum size for a rear fog lamp?
just thinking of a way of mounting rear fog into bumper but not the usual square halfrauds type. seen some nice chrome ovals on ebay but may be a bit small??
also seen some chrome oval spots the same size which i want to mount for work lamp / bright reverse lamp for reversing down my dark drive at night