Shivering Isles (both the location and the expansion overall) was bigger than Morrowind's Solstheim/Bloodmoon. I can't speak for Dragonborn for i don't have it but the way i hear it, it isn't that large.
I doubt that Redguard will be any bigger than Shivering Isles, either in playable area or content overall, but considering how long they've been silent about next DLC, i think it will be big.
As for its location, i can think of a few possibilities: Stros M'Kai but considering that the island is smaller than Solstheim, it alone is unlike. Stros M'kai AND some area from Hammerfell is a possibility.
Alternatively, maybe some area of Hammerfell, blocked by natural obstacles, Thalmor border or something.
By the way, I wouldn't be surprised if the DLC was about Redguad versus Thalmor situation.
One possibility would be the Hammerfell/Skyrim border, though i'm not sure there any routes beetween Skyrim and Hammerfell there. Unclear maps.
What i'd like to see is Orsinium and area around it, the bay beetween Hammerfell and High Rock. Not sure how Redguard would fit that really, but... close enough