I am a Feudal-era peasant in England. Do I know how big Britain is? Have I ever seen a map before, and if so, was it of just the country, or of Europe?

Could OP please give a more specific time period? I suspect there is a big difference between what a Mercian peasant living during the Heptarchy would know of Europe, and what an English peasant living in the heart of the Hundred Years War would know.