I define a micro-adventure as a 1 to 4 encounter/room exploration. With some systems, that can fill two, three-hour sessions, but I'm focused on the OSR's constellation of gaming systems. This is based on my several years of being the initiative keeper for our group and combat usually lasts around 4 rounds. If it's not over in the 4th round, it's pretty well determined who's going to be taking the dirt nap.
I have a handful of adventures floating around, but the one that seems to get the most play, from what I read is The Salt Pit from The Manor #1. It covers a little over 4 half-pages and that includes my masterly drawn map of a hole in the ground. There are 9 detailed areas. And 1 monster. Done.
The simplicity and the brevity of descriptions are the selling points of a micro-adventure. A GM can read through it in a couple minutes and run it. And most OSRians are masters of last minute alterations.
Present a situation. It shouldn't take more than two sentences. Three, if your feeling verbose. Paco and his family work a small salt mine, but recently some creature has moved in. They need your help to get rid of the creature so they can get their quota for the local lord. Paco and his family could lose everything.
Simple and to the point. It might not motivate everyone, but it's presented in very simplistic terms with logical consequences. The party can chose to run with it or hit the tavern.
When describing encounter areas within a micro-adventure, keep it brief, but personal. You could write that the mine is covered with wooden planks. But it doesn't push the adventure along. Paco built a cover to the mine to keep his children safe. The trapdoor he installed as an entrance has been ripped off its hinges.
Each encounter area doesn't need to be a result of the creature, it could be nuances of how a salt mine works. Don't be technical, just add an interesting detail. These small details can make a big difference from a micro-adventure being interesting or sucking.
The final encounter with the creature, the focus of the adventure, should have been developed through the other encounter areas. A 'build' to the final battle.
Final encounter...make sure to use the creature's abilities to the fullest. Make it as dangerous as it is built to be. If there is no sense of danger than the adventure will fail. There's little a GM can do if the party starts slinging out criticals and/or doing max damage. But, until that happens make them nervous. With the final battle in the Salt Pit, the creature uses its chameleon power to possible gain surprise and it's odor to weaken the party before it attacks.
This is what has worked for me in the past. My adventure Where is Margesh Bloackblood? is another example of how I develop a micro-adventure. That one has a slight twist to it where the GM rolls randomly where Margesh can be found.
One last note. The map(s). With micro-adventures I usually keep them bare bones with boxes with numbers in them. Symbols for doors (possibly windows). I want to be able to look at the map and memorize it in a glance. But some folks with an artistic flare can develop a micro-adventure with just the map. Look at many of the one entered in the one-page dungeon contest.