What is the minimum meteor size to see a falling star? When a meteor is going through our atmosphere you will see a light beam made by the gas the meteor compressed. But what is the minimum size to see a 'falling star'? Is a 1 mm3 meteor visible?
The International Meteor Organization (IMO) defines a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than a asteroid and considerably larger than an atom or molecule." This definition is further refined by Rubin and Grossman, who state "a meteoroid is a $10$-$\mathrm{\mu m}$ to $1$-$\mathrm{m}$-size natural solid object moving in interplanetary space." Additionally, the same authors consider a defintion for a micrometeoroid, which they define as "a meteoroid $10\hspace{2pt}\mathrm{\mu m}$ to $2\hspace{2pt}\mathrm{mm}$ in size." 

According to astronomer Karen Masters from Cornell, the typical meteor has a mass of a few milligrams. The American Meteor Society (AMS) also states that the most-commonly observed meteors are roughly the size of a grain of sand. This would seem to imply that many meteors are smaller. In fact, this is also implied by the various statements by many groups, including the AMS, IMO, and IAU, that micrometeoroids commonly cause meteors. Because of this, I would argue that a meteoroid with a volume of $1$ $\mathrm{mm}^3$ would easily cause a relatively bright meteor. Statements surrounding the frequency with which micrometeoroids cause meteors would seem to imply that, on average, any body larger than $10$ $\mathrm{\mu m}$ will be observable as a meteor. These figures will vary largely depending on the entrance velocity of the object.