Hard Drive Enclosure

I have an 80GB 7200 RPM Ultra ATA hard drive in my old XP computer. As best I can measure, it is 4.25" x 6.0" x 1.0". I would like to put the old HD in a eSATA/USB enclosure to use as an external drive with my new Vista computer.
What size enclosure would I need and recommendations as to enclosure to purchase
JimK

That is what is called a standard "3½" drive". You just need an external enclosure for that size. BUT there are two interface questions to answer in selecting an enclosure, and they are SEPARATE questions. One is the interface between case and computer. In your case, it appears you want one with dual options - both eSATA and USB2 - which is a very good choice. The second question is the internal interface between case and hard drive. For what you have now, you only need a case that accepts a common IDE drive, although these also are known by "ATA" or "Ultra ATA" or "PATA". You do not need a case that can also accept a SATA drive now, although you might find that extra capability useful later if you decide to change what drive unit is mounted inside.