One of the more deceptively confusing aspects of the Kirby series is Kirby's size. Many sources report that Kirby is supposed to be 8 inches, or about 20 centimeters, but there is all sorts of information that seemingly contradicts this.
The following is a recap of as much information I can find either supporting or challenging this.
The best known source for the "8 inches" measurement comes from the Super Smash Bros series. In the 1999 original, each character had their own description, with Kirby's stating "The 8-inch high Kirby hails from a distant, peaceful star." This remains true in the Japanese version, although the 20 cm figure is used rather than inches. Melee continues this in its trophies, stating that Kirby is "only about eight inches tall."
It would be tempting to discount these excerpts because they're from a non-canon crossover game. However, since they were developed by Masahiro Sakurai, Kirby's creator, and HAL Laboratory, the primary developer of the series even at that time, these sstatements should be given more weight.
Conversely, it is worth noting that the Super Smash Bros series stopped referring to Kirby's exact height after Melee. Though heights are rarely used in trophy descriptions from Brawl onwards, they do appear for characters from Custom Robo and Chibi-Robo, which are measured in inches.
In the leadup to SSB4, Sakurai made regular Miiverse posts. On 2013 August 7, they reconfirmed Kirby's 8 inch height in a post discussing character scaling. However, the same post was later replaced by one showing Luigi, who was first confirmed for the game that same day as part of a Nintendo Direct. I don't know whether this in any way decreases the original post's validity.
In the "Chop Champs" subgame of Star Allies, each chunk of log is 25 centimeters tall, and is slightly taller than the nearby Kirby.
In Canvas Curse, the goal game measures distance in inches, and by comparing the size of Kirby in this game to the 50 and 100 inch markers, Kirby seems to be a bit over 19 cm.
Though it'd be tempting to scale Kirby against other objects to determine his size, the fact of the matter is that the fantastic nature of the series makes usually makes this impossible. However, there are several parts of the series that suggest Kirby is much taller than 8 inches.
First, humans are seen at a couple of points in the Kirby series. In Dream Land 3, Kirby encounters Ado (or Adeleine) and Samus Aran, a Metroid cameo. In their respective sprites, Kirby is 18 pixels tall, Ado is 29 pixels tall, and Samus is 48 pixels tall. If Samus Aran's height of 1.91 meters is used, which was canon even at the time, it would make Kirby roughly 71.6 centimeters tall and Ado 115 centimeters tall. That would be a reasonable height for the child Ado, but would make Kirby much taller than stated elsewhere.
Adeleine's height can't be ignored as a single game's oddity. They return in The Crystal Shards, and often appear near Kirby in cutscenes. In this game, it is harder to compare heights directly due to the game's 3D nature, but Adeleine seems to be somewhat less than twice as tall as Kirby. The same applies in the recent Star Allies. Again, even if she is short for a human child, this would put Kirby close to that earlier 71 centimeter mark.
Speaking of The Crystal Shards, consider Shiver Star. Shiver Star is heavily implied to be a post-apocalyptic Earth, based on its very similar continents, single rocky moon, and abandoned stores and factories. When Kirby enters the front door of a mall, he is roughly a quarter of the door's height. Doors to commercial buildings are usually 80 or even more inches, meaning that Kirby would have to be at very least 20 inches, or 50 centimeters, for this to make sense.
To Do - City Trial's "Race X Miles" achievements, game manuals post-2000,
Love and tolerate.