Thermals suck. You need to enlarge your designs and provide heatsinks, even if they're PCB-heatsinks. (The copper itself on the PCB-board can be a heatsink, as long as you make the copper-fill large enough, it will radiate the heat out and safely lower the temperatures of your design). But you can't do that if you're trying to prove a point with impractical 5mm x 5mm designs.

Heck, even a real-world 8mm x 23mm heatsink (https://www.digikey.com/en/products/detail/aavid-thermal-division-of-boyd-corporation/573100D00010G/5873431) is specified at 0.8W @ 30C rise. Like... a real heatsink (not a PCB-copper fake-heatsink because I'm being cheap). I have no idea how bad a 5mm x 5mm design would be.

This design is probably fine for... I dunno, 30mA or something (0.1W or less), which is probably possible and the "intended use". Well, really, the intended use is to just prove that you've made the smallest board without any actual practical consideration it seems, lol.

I dunno, I don't have any FEA software to simulate the heat-flow of a design like this. Just kinda guessing at it.

But yeah, the practical considerations of these tiny designs are... well... impractical. There's a lot of benefits to keeping things at the ~60mm x 20mm size (aka: a AA cell or so).

Like, how do you even power this 5mm x 5mm thing? CR2032 works but those cells are kind of a pain in the ass cause they're so small.