I wish to install a taller stem on my Nishiki Cresta, 1983 model. When I've looked for stems available on-line, the size is stated in millimeters, 22.2, for example. There is a 60 on my stem, which I assume is the length of the stem. The only other number is 5355, stamped near the bottom of the stem, which appears to be the model number (SR product). Are stems from the era of my bike standard? Do I need to just take the thing into the lbs...actually the one that's 30 miles from home as the two nearest me don't impress me as knowing much about bikes older than 5 years, or does anyone know the diameter of the stem on the bike?

most stems come with quills in two sizes: 22.2 and 22.0 mm. The latter is specific to french bikes, and the first covers all other 1" threaded fork steer tubes. I would consider the length of the stem and the diameter of the stem clamp (where your handlebars slide through when installing them).
Nitto makes real nice tall stems, called the Technomic.

Don't forget the 21.15mm size.
But, that Nishiki should have a 22.2mm stem, otherwise known as a 1" stem. Not to be confused with the 25.4mm (1") diameter stem that fits into a 1 1/8" steerer. The bar clamp size should be 25.4mm, the 60mm is the extension.

There is a lot to consider when buying a quill stem, you need to make sure the three sizes are correct, plus the height.

The seat post is at the maximum level...original seat post...which is right for me. The bar, though, is about a 1.5" too low when the stem is set at maximum height. Any suggestions on the length of stem I should buy? The frame is a 21" and, though I'm 5'9", is a good fit as I have a fairly short inseam for my height.

hard to do a fitting at "long distance", but you clearly should look for a stem with a 1.5" taller quill (such as the mentioned Nitto Technomic) or perhaps one with a positive angle upward (such as the Dirtdrop shown below). As to the length of the reach, you can get close by lowering your saddle to be level with the current 60mm stem; then sit comfortably on the saddle with your hands on the drops of your handle bars; take a sighting of how the centerline of the bars line up with the CL of the front wheel axle, estimate how much longer the bars need to be moved forward (I'd guess forward) in order to block the axle: that's the ROT (rule of thumb) as to how much longer your stem's reach should be. Remember, it's just a ROT, and YRMV.