@mntmn I'm a big fan of the mini-business desktop form factor (see https://www.servethehome.com/hp-elitedesk-800-g4-mini-tinyminimicro-guide-review/ ). I own a few of them and the design is fantastic. They have a design like a miniature rackmount server (heatsink on cpu and separate blower fan), and from my experience they are super easy to work on. Also, the size is probably more than large enough for a 2.5" sata drive (if desired). Also, they stack nicely.
@mntmn I don't think there is a spec per-se, I think it is more of a form factor (e.g., a mid-tower). That said, they are often referred to as 1L desktops as they occupy roughly that volume. This page might be a good inspiration for the dimensions of the board and enclosure though:
https://www.asrock.com/nettop/AMD/Jupiter%20X300/index.asp#Specification
@leimon do you know if there is a spec/standard for these? i.e. at least measurements, mounting holes etc?