I’ve seen the term and don’t know what’s specific about it.
Most “ashcan” collectables seem like ordinary comic books.
Ashcan editions are normally giveaways at conventions or stores, printed on cheaper stock paper, usually in black and white. Kind of like a work in progress. Since they don’t produce as many as they would in a regular run of an issue, they sometimes become quite the collectible.
The definition I saw a couple weeks ago is that they are the first publication in which a character or concept is printed on in order to secure the copyright to its name. Just a few are printed, and most are thrown away to remove them from circulation - its sole purpose is for the copyright.
Studio Foglio recently printed what it called an ashcan edition of its risque xxxenophile comic book to raise money. It was a collection of drawings that were never used, plus a 5-page comic, plus some other stuff.
So “ashcan edition” in this context could be a one-off comic that’s not meant to be considered part of the continuity. Stuff pulled from the ashcan, you might say.
Most of the ashcans I saw back in my collecting days were smaller than comics, somewhere between digest sized and regular comic book size.
Securing copyright by publishing something isn’t really necessary as it will be copyrighted as soon as it’s made. Most ashcans as I understand are, as stated, little previews that are given away at conventions. They often appear in Wizard magazine.
Generally, for indie comics, they’re simply printed on 8.5x11 and folded over, although there’s really no standard size.
In the end, it’s really whatever you call an ashcan (kind of like a 'zine that way).
It isn’t necessary now, but it was done to protect copyright in the past, most famously with 1978’s Cancelled Comics Cavalcade