The planet (TOI-6894b) is a low-density gas giant with a radius a little larger than Saturn’s but with only ~50% of Saturn’s mass.
IIRC Saturn has very low density, was it lower than water? (IOW it would float) so this would be even lighter.
I understand how they measure mass, but how do they measure an exoplanet's radius, especially to that precision?
Not sure in this specific case, but usually by measuring the brightness of the star as the planet passes in front of it, like a partial eclipse. That’s why most of the planets we have discovered are giants.
I did not read this paper, but typically the diameter can be inferred by the transit time.