Question : Problem: How big a 2nd hard drive can I install?

What determines the size and/or type for a SECOND hard drive to be used for storage (besides having an open bay).

Answer : Problem: How big a 2nd hard drive can I install?

Sorry...I seem to ramble in this answer, but the most helpful info is probably at the end.

Your motherboard model would be most helpful.  If you have a name-brand (ie. Dell, Gateway, Compaq, etc), the system model may also help in finding out what the max size of a hard drive can be.  

However, one can usually guess based on system specs.  For example, if your system is an early Pentium II, I wouldn't trust that the system can handle anything more than 32GB.  Anything a little newer can generally handle up to 120GB or so.

Beyond that, even if the system is very old and you want to put, say, a 250GB drive in it, most hard drive manufacturers offer a free hard drive setup utility that can augment the system BIOS, allowing you to install a larger drive than your BIOS can support.

Here is a link to Western Digital Data Lifeguard Tools
http://support.wdc.com/download/index.asp#dlgtools

Using that utility, I once installed an 80GB drive into a system that could only handle 32GB natively. It worked like a charm, and it even cloned the old hard drive to the new one for me.

So I suppose the final answer to your question is that if your BIOS limits the maximum size of the hard drive you can use, manufacturer software can usually work around the limit, allowing you to use the full capacity of the hard drive.