It was smaller than the iPhone 8. It was slightly thicker (0.01"), shorter (by 0.27") and narrower (0.13"). And it had a bigger display despite being overall smaller. It was bigger than the 5, though.
I did, however, take a series of photos comparing the Mini to the SE- and the packaging of the 6 (I think I gave the phone away but still had the box).
https://imgur.com/a/iphone-mini-vs-iphone-5-vs-iphone-6-case...
The SE was the one that shared the 8's dimensions, the Mini was smaller than both. That Mini appears to be in a case, so the comparison photos aren't as useful as they'd be otherwise.
(Also, it looks like I couldn't do arithmetic earlier or typo'd the number, the Mini is 0.12" narrower, not 0.13").
Maybe because it's the dark version of the phone you can barely make out the shiny apple logo, but it's there, you can see the metal edge of the phone.
It's not in a case.. obviously, that would be a stupid comparison.
"SE" is a nondescript product line and the only physical differences are the tone of the chassis and a stamp; it's a fair representation of the size of a 5.
Apple was the last hold out with small phones, years after everybody else moved to huge phones.
But even Apple couldn't resist the pressure from the consumers, and eventually yielded.
And somehow we ended up with more Pixels and not minis.