The Sting Standard is truly standard size, as in the standard size of wood racquets of the time. Here's mine, compared with one of my Sting Midsizes. The Midsize is 85 sq. in. -- the Standard is probably in the 75-80 sq. in. range. The Sting Midsize came out in about 1985, but I can't find any mention of the Standard in Kuebler's Book of Tennis Rackets or any source online. My guess is that the Sting Standard predated the Midsize, probably in the early '80s, as an alternative to the Ultra, which was much more expensive.
I'm a big fan of the Stings, and the Standard I have is the only one I've ever seen in searching for Stings in the past few years, so I don't think there are very many of them out there. they have the same graphite construction and that distinctive curved throatpiece of the Midsize, so the only real difference seems to be in head size.