How big is a slice of bread?

Well that depends on the type of loaf the slice was cut from and the angle of the slice as compared to the long axis of the loaf.

But with shop purchased pre-sliced bread the dimensions are ROUGHLY 10cm * 11cm and up to 1cm thick.

[The law usually relates to the overall weight of the bread loaf being sold rather than to the dimensions of the baking tin/loaf so I don't think there is a standard for a slice]