Skyrim isn't all that big - it is made artificially big by potatofying the landscape, making any journey impossible with a direct path. It also has fake coordinates so that it is '37km^2' in the ingame units, but these do not translate into real dimensions that match the visible vertical scale of objects - when this is taken into account, and measurements checked against other metrics (travel time, door height etc) the size is effectively only around 5x3km.
I think the area inside the fence in the beta was less than 300 ha (3 sq km) based on scaling from the map extent and an approximate trace of the boundary. This based on scaling estimates of the map screen shots that give approximately the real world distance Talmberk > Samopesh. There was around the same amount of filled in terrain outside the border, but over half of the map was blank and not detailed at all on the ground.
A map of similar size would be the Witcher 3 Velen/Novigrad map, but the scale of individual features more closely matches the White Orchard map with fewer larger settlements with realistic agricultural support around each, rather than the rather cramped Velen map with too many settlements and hardly any tilled land compared to the population sizes in most cases.