A new page: http://www.bushfarms.com/beesspace.htm
This may be the hardest and perhaps one of the most important concepts to grasp in beekeeping. Having the ideal amount of space is a tricky thing to manage but it makes the difference between prosperity and poverty for a honey bee colony. I used to find it very confusing when people would talk about always keeping strong hives. It seemed to me that a nuc or a split was always weak, by definition, but I will offer a new definition. A strong colony is merely a colony with a good density of bees.
Once you make this paradigm shift it becomes easy to maintain strong colonies. Any time you see a colony struggling, compress them. Put them in a smaller space. Remove any combs they are not occupying. Remove any combs they have lost control of as evidenced by small hive beetle or wax moth larvae. Freeze those and don't give them back until the colony has grown enough to manage them. An empty foundationless frame is better than an unoccupied drawn comb when you have an issue with hive strength. I call this process "compressing the hive". If you make the hive smaller and increase the density of the bees you will find that a struggling colony is suddenly a booming colony. It's like they were living in a house that had too high of a cost and now they are in one they can afford. Granted they are "affording" it because they have enough bees to do the work, but still they are not overwhelmed by the space that they have to heat, guard and cool. I have seen many a struggling colony turn around quickly when put in the proper space. Slightly crowded is best other than in the main flow when you have to work to keep the space open.
Fear of Swarming
The other issue, of course, is the concern that usually caused the problem of too much space and that is the fear of swarming. Beekeepers often give an overly generous space in order to avoid crowding that might cause them to swarm. When you have a colony that is really exploding and the nights are warm and there are plenty of resources coming in, then it's hard to put too many boxes on, but often these are put on too early or left too late. Whenever you see a colony struggling, one of the first things I would do is give them less space.
Nucleus hives are handy for this purpose. Eight frame mediums are also handy in that they are half the volume of a ten frame deep, so you have more ability to adjust the space to be "just right". If I have two frames of bees I like to have them in a three frame nuc. If I have six frames of bees I like them in an eight frame box.
Winter is another time that just the right space is what you want. I know you'll hear all these people say "the bees don't heat the hive, they just heat the cluster" but I'll guarantee you will be warmer in a small room than a large room when they are cold and both the same temperature. I have spent a lot of my life working outside or semi-outside building houses and little things make a big difference when it comes to cold. I want my hives going into winter with the space they need, not a lot of extra space. Any extra space, if necessary, should be on the bottom. This is part of the concept of overwintering nucs. A small cluster of bees can get through the winter if the density of the bees is high enough.