For NT 4.0, the size limit of the boot partition (C: drive) is something like 7.8GB, provided that it's formated as NTFS.
Note that you can also potentially have the WINNT directory in a drive other than C. In that case, you will have your boot.ini, ntdetect.com, and ntldr in a very small C drive and the WINNT directory in D drive (the rest of the hard disk) or whatever.
If your hard disk is less than 7.8GB, I'd prefer to partition the whole thing as one drive. The standard NT setup program will not allow you to do that. It will limit the partition size to 4GB (even if you specify NTFS) because this setup program is limited in the sense that it has to initially format your partition as FAT and then convert it to NTFS. Since FAT cannot be bigger than 4GB, it would not allow you to specify anything bigger than 4GB.
The 7.8GB limit is the limit imposed by the BIOS int13 interface. Once NT is running, it does not need the BIOS to access the disk. So any partition (NTFS) that is not the boot partition can be as large as you want (I think the limit is in the range of TB or EB). The problem is the critical NT boot files are stored on the boot partition and you have to read from the boot partition before you can run NT. That's why the boot partition is limited by the BIOS int13 interface. I think Linux has the same limitation too. The newest OSes like W2K use the newer "extended int13 interface" that does not have this 7.8GB limit.
To format your C drive as 7.8GB, you have to use some other way to get a 7.8GB partition and then format it as NTFS, before you run the setup program. You can do this by physically attaching the hard disk to an existing NT system as an extra drive and partition/format it there. Or you can use Partition Magic, I guess. Note that this partition must be the first 7.8GB of your drive. That means you want it to be your first partition. The exact limitation is actually expressed in terms of # of cyclinder, # of tracks, and # of sectors. You have to make sure you are within the limit. I think Disk Administrator will warn you if you exceed this limit. When in doubt, find a reference on the int13 interface.
Once you have such a partition formatted as NTFS, you can put the hard disk back to the original PC and run the setup program. When it asks you where to install NT, choose the partition you formatted and make sure you choose to "leave the existing filesystem intact".
If this is not complicated enough, there are also bugs in the IDE driver (ATAPI.SYS) in the NT CD-ROM that may make your installation ultimately unsuccessful. To be absolutely safe, I'd make sure "the existing NT system" that I format my drive on has at least service pack 4. And when you run the setup program, use the boot floppies instead of the bootable CD. You will then have a chance to select a custom IDE driver instead of using the buggy one on the CD. The custom IDE driver you want to use is available as a downloadable package from Microsoft called "Service pack 4 ATAPI driver".
The other person mentioned the use of Partition Magic to format a boot partition larger than 7.8GB. That will initially work but can fail a year later mysteriously. The thing is the critical boot files (boot.ini, ntdetect.com, and ntldr) in the boot partition must be located in the first 7.8GB of the drive to be addressable by the int13 interface. If your boot partition is larger than 7.8GB, your critical boot files might be initially in the first 7.8GB but later be moved somewhere else (by defragmentation for example). When that happens, your system will fail to boot.