Using the power of digital callipers and third-party power supplies… too bad I didn’t have the first-party supplies on hand. That’s probably why my PAL inner-barrel measurement wandered by about 0.1mm.

Super Nintendo (PAL unit measured here)
Inner barrel: approximately 2.5mm
Outer barrel: 5.5mm
Standard PAL supply: AC, so polarity does not apply

Super Famicom (NTSC-J)
Inner barrel: approximately 2.1mm
Outer barrel: 5.5mm
Supply: DC
Polarity: centre-negative

The important correction is that the similar-looking connectors do not mean the PAL Super Nintendo and Japanese Super Famicom should be treated as having the same power requirements. In particular, my original “centre pin: negative” line was wrong as a general description of the standard PAL AC input.

Come to think of it, I don’t think I’ve ever held a Super Famicom or Super Nintendo power supply. What strange times we live in! :)

Sources