[Kereth]

When size is enough and too much varies between certain areas.

Unique rooms:
A moment for definition- a "unique room" is not a room that doesn't photocopy code from another room, it's a room that has significant purpose in the game and that must be visited fairly regularly. These should be at least relatively low in number, because gameplay may, especially in RP MUDs, rely on players running into each other here. Few people play a MUD with the intention of spending time alone. If they wanted to do that, they'd play something offline. Your number of these rooms can be much higher in a less RP oriented MUD, where players can meet with out-of-character communication on channels and things like that, but if you're going to run an RP MUD, ideally, you might even want all these rooms grouped in a central hub, so that players are basically thrown together as a matter of every day life. (E.g. Having one room in the entire universe where you can buy and sell things. Annoying as a game mechanic, but it WILL make characters meet each other. Not ideal, but a possibility)

Overworld:
Without a map, the overworld becomes somewhat limited. Valid overworld size increases with a map, but it always needs to be traverseable. How traverseable terrain is is measured in movement time, not room size. If your MUD limits how fast commands may be processed to enough of a degree, even walking 20 rooms can be tiresome. If your MUD makes your character stop to rest every 10 rooms or so, that becomes even worse. Whether your MUD incorporates flight, teleportation, or other transportation systems, can also be an issue. You can have a truly massive overworld, as long as getting to places on it does not take an extended period of time. Then again, some MUDs seem to be built to have travel over long distances feel like just that: long travel. The gameplay result of this is that players will congregate near the central hub and areas placed in "foreign parts" will be almost unpopulated. Players will tend to take up residency in certain portions of the game and may not come to interract with others outside their desired play field. It takes a healthy player base to support this, but DiscworldMUD has one of the slowest, most complicated travel experiences I've seen on a MUD, yet it still manages to be healthy, while games like NW, which have a more compact field to force characters to interract, or RetroMUD, with an expansive world(s?) but very quick movement and more plentiful sub areas, are similarly functional.

Areas:
Quality over quantity first, in areas, but after that, quantity is not a bad thing. For RP MUDs, or others lacking or restricting out-of-character meetings and socialization, a strong central area to throw people together is important, but many players, even in those games, will be thrilled by the allure of more and more areas to explore. Number and variety of areas helps ensure that a game is enduring, but one needs to be careful that areas are always unique and well thought out. Buggy areas, boring areas, and unbalanced areas are worse than no areas at all.

In summary, when MUDs come across as "too big" it's usually a problem in one of these three areas. Keep a good tab on each (minimum number of unique rooms as potential meeting places, an easily traversable map, and quality areas) and you should be fairly safe.