It sounds like you want all your users to only use a certain amount of the drive, so create a /home partition of that size.
If you don't have a separate home partition, then it becomes a standard directory in the root partition, like any other. What happens when you create a separate home partition is that there is a directory in the root partition called home, and any interactions under this directory get redirected to the other partition. This can be confusing, since its hard to casually tell the difference between the two, because of the seamless integration of the filesystems. If you were to unmount the home partition, it would simply appear to be an empty directory.
If you were to put this into DOS/Windows terms, say you have root partition c:\. You also have a directory c:\home. If DOS/Windows did with the home directory was the same as a separate home partition in Linux does, then you have directory c:\home, but if you did a dir/w, or looked at c:\home in Explorer, you would see the files in d:\.
In Linux you never really see what partitions you're using, because partitions simply "take over" other existing directories to make themselves visible.
Lets look at a simple case with a barebones setup. First you need a root partition, because in the beginning there is only the "/" directory, thats it. So, you tell your first partition to take over "/" Okay, well the first partition has a directory called home with no files or subdirectories. Here comes the second partition. You tell it to take over (mount to) "/home". Now, we create a user iwantlinux. The adduser tool then tries to make a directory called /home/iwantlinux. The second partition says, "thats MINE" and makes that directory in itself. There is only a iwantlinux directory on the second partition, not the first.
Now, if you then told the second partition to bugger off (umount). Then there would just be the empty /home directory, with no /home/iwantlinux directory, because when the second partition went away, it took that information with it.
Lets say now that you just had the first partition mounted to the "/" directory, and no second partition. Adding the user would create a /home/iwantlinux directory under the first partition like usual.
See how that works?