Medieval Food Question(s)

I have a friend who's currently transcribing a treatise on horsemanship (Gervase Markham), and it has some terms she's having trouble figuring out.

One is comparison of size of some horse thing to breads.

Does anyone know what the size of a halfpenny wheaten loaf, or a penny white loaf is? (Not sure if these are two ways to say the same thing or two different things).

Any recommendations on sources for finding this sort of thing?

Thank you!