Typically, a shield should cover your whole torso.
Shields seem to have been used universally by all warriors.
From the first to the tenth century round shields seem to have been the norm, being either flat or 'watchglass' shaped in cross-section. They are always shown with a boss and often have wooden or metal bands on the back to strengthen them. All the examples found have been of planked construction although there is some evidence to suggest a plyed construction would make the 'watchglass' shape easier to make. Some shields were edged with a rim of sewn thick leather or hide to strengthen them whilst others were possibly faced with leather or rawhide. However none of the shields excavated demonstrate this even though 'leather covered' is a phrase writers used from that period.