The 271 is roughly 3" long, maybe 1.5" front-to-back. My most-used small router plane is actually a patternmaker's copy of a slightly smaller router, done in cast iron.
The Stanley 271 seems to be on-again, off-again in production/availability. Veritas makes or made one about the same size, with an arguably-better blade holding mechanism. Lie-Nielsen makes a small router plane that is about an inch longer.
If you buy new, cost is going to be about $75-85 for a Stanley or Lie-Nielsen, or around $55 for a Veritas. Unless you get lucky, a used one won't be much less than those prices. If you watch on ebay, you'll sometimes see patternmaker's re-casts in brass/bronze/iron of a Stanley 271 or similar small router plane. Most of these seem to work as well as the originals, and typically sell for under $20--at that price it is worth buying one just to try out. Someone seems to be doing an occasional run of really small--2 1/4" long--router plane castings in bronze, but I haven't been lucky enough to purchase one or locate the maker.
Older router planes often come missing irons. I usually make replacements from old Allen wrenches: the steel is usually good, and the flats help the set-screw hold the iron in place.
