I believe around 450 sq inches of floor space is considered bare minimum in the USA and already way better than what you see in stores, so it's a very good start.
However, my dwarf's
DIY cage is about 840 sq inches and, honestly, I'd reeeeally like it to be bigger. He's so full of energy and likes to run and explore, and I believe all dwarfs are like this. Plus, I'm pretty sure you'd want to add tunnels and hides, and toys, and puzzles, and bridges, and so so many other things to provide enrichment... And find yourself not being able to fit everything in 450 sq inches
So I'd suggest to go even bigger
if possible.