How much does a tuppenny loaf weigh?
Depends when.
https://en.wikipedia.org/wiki/Assize_of_Bread_and_Ale
The price directly related to the cost of the wheat so would have fluctuated over time, and even where the bread was sold.
There were also several types of loaf. I think this would typically refer to white bread, the most expensive.
This is from 1765 in London

I'd say around 'one pound', but it's massively variable and not really answerable without much more context.