According to the in-game data, its height is 23'0" and its weight is 880.6 lbs.
Like other Glitch Pokémon, a (italicized hereafter for clarity) shares a Pokédex number with Poliwhirl. Because of this, a shares base stats, its type (Water), and a moveset with Poliwhirl, though a can also learn glitch moves.
a's sprite consists of misread data; this is a trait that many glitch Pokémon share. Its name often consists of a lowercase "a" followed by several screens of glitched tiles that take several button presses to scroll through; because of this, it is often recommended that a player nickname a after catching it.
a is generally stable and rarely causes lasting damage to save files. There are a few potential hazards, however.
If a is raised to either level 13 or level 42 and stops leveling up at either of those levels, then the game may freeze or the screen may go blank. Like many glitch Pokémon, a may also attempt to learn glitch moves such as "TM25" (which is not TM25 "Thunder" -- the attack is actually named "TM25"); teaching or using these moves can result in a range of unpredictable effects, including crashes.
Like the vast majority of glitch Pokémon, a can not be obtained by normal means. It can only be battled with the aid of a cheat device or the third method of the Mew Glitch (using a Special stat of 192).