There is no qualification for acceptable size.
The question is one of whether you can keep the size entertaining.
How fast can one move through the world?
What is the scale of your interaction with the world?
Is gameplay dependent upon location or does it occur wherever the player is at?
How much content do you have in the world?
Are there things that make the empty world fun in and of itself?
Oblivion is a good example of failing on all counts. The only thing you really do in much of the world is walk or ride toward something interesting. Nothing makes most of the world inherently fun and it is, very often, completely pointless because you're just going to fast travel anyway. There isn't enough raw content to justify the size and the content is tied to particular locations. The main point of the size of the world is simply to give you a strong feeling of scale. This is not necessarily bad, but they could have made much smarter use of that scale (see Morrowind below).
Some games that do better:
Minecraft is a great example of a game that can get away with an infinite world because gameplay happens crucially wherever the player is. You don't need to travel to find something to do. The random terrain generation also means that exploration can yield new and interesting landscapes and, statistically, will do so at pretty regular intervals (compare to the bland filler space often used simply to make a setting feel big).
Rockstar gets open world games. GTA gives you a way to get around quickly in a way that can be inherently fun and a sandbox that gives you fun things to do outside of the location-dependent missions. RDR removes some of the ability to create chaos for entertainment, but gives you side-goals involving the space, gives you a way to traverse it quickly, and gives you random things to do along the way from seeing a horse you want to collect to rescuing a man chased by a pack of coyotes. Again, missions are tied to locations, but many of the missions also make use of the large landscape too (the mechanics allowing for a lot of gameplay to take place while traveling are pretty brilliant). RDR has quick travel, but I didn't end up using it a single time before finishing the main storyline (only once I had started trying to finish a few leftover gathering tasks). This was not a matter of pride - this was a matter of wanting to ride around to each mission and objective.
I would say Morrowind also does a very good job too. The roads on the island are twisted enough that the world seems quite large and travel times can be relatively long and there are huge stretches with very little to do but walk and there's very little fun inherent in traveling. The solution, however, is the fast travel system. By creating an in-world fast travel system built on a web of subsystems, they require that you learn to navigate the system and gain access to some of its parts through certain achievements in the game (finding the indices, buying and becoming capable of casting travel spells). You earn the ability to travel more quickly and avoid the pitfalls of the usual meta fast travel systems. The ability to traverse the world quickly becomes a reward both for your achievements in the game and your ability as a player to navigate. And the game also makes great use of this fact later on in the main story as it forces you to move farther and farther from these networks, creating a greater sense of isolation and remoteness. In short, it takes the problems it has and turns them into strengths.
Prototype and Assassin's Creed also deserve mention as games that do a great job with a large, mostly uninteresting city by making the method of moving through it itself quite satisfying.