Assuming that a ‘two penny loaf’ is the same as a ‘tuppenny loaf’, it likely depends on the time and place, for example, in Dartford Prison in 1788 it was 19 ounces:
allowance, a two penny loaf a day (weight 19 ounces)
But in 1774 at Norwich Castle county gaol it had been 20 ounces (source).
In April 1766 the ‘Assize of Bread in and throughout the County of Leicester’ a two penny wheaten loaf was set down as 1lb 5oz, or 21 oz. (source)
Shrinkflation isn’t new.
In the children’s game of leapfrog, according to wiktionary, to ‘tuck in your tuppenny’ is to tuck in your head ‘perhaps named from a tuppenny loaf’. From which we might assume that a tuppenny loaf is head-sized, or child’s head sized, or that it was a rhyming slang for ‘tuppenny bread’ rather than loaf.
In 1728, according to Eliza Smith's The Compleat Housewife
it was the amount of bread upon which, thinly sliced, you might spread a pound of butter. (Source)
So somewhere just over half a kilo or about the size of a head, depending.