I agree with everyone here, but that aside...
I feel that the size of an open world should be linked to how quickly one can traverse it. Most obviously, games with vehicles like cars and planes will need bigger game worlds than the likes of Elder Scrolls games.
In Zelda U, the main methods of travelling seem to be going by foot and riding Epona the currently unnamed horse. Assuming Epona is not hard to get access to, players will coover the most distance on horseback. Assuming the game uses not entirely unrealistic horse speeds, Epona will travel at a rate of 25 miles per hour, or 40 kilometers per hour for you non-Americans.
The following is a list of some games' approximate area in square miles (lovers of kilometers should multiply by 2.5 or so):
- Batman: Arkham City - 2
- Skyrim - 15
- GTA5 - 50
- Far Cry 3 - 300
If this batch of games seems eclectic, that's the point. Obviously I'd want a bigger map than Arkham City, which features no real vehicles. But Skyrim, despite the less accessible horses and rough terrain, has fast travel. GTA5's world has vehicles that are much easier to get, go at speeds over 100 MPH, and in some cases fly.
Based on these thoughts, I think Zelda U's map should not have a much larger world than Skyrim. Because of how math works, areas become larger disproportionately quickly. If you wanted to take Skyrim's map and re-scale the world so journies took 50% longer, the map would grow by 125%. So I feel that Zelda U should have a map somewhere between 20 and 30 square miles, assuming that most areas are fairly open and encourage direct travel via Epona.