There are lots of planets in the cosmere. Some of the worlds feel "larger" than others. However, it's not clear which one has the largest population, or how it might compare to the population of Earth at an equivalent technological level.
Roshar seems like a good candidate for the most populous planet, since it seemingly has dozens of countries; on the other hand, they're almost all on one continent. Maybe it's Sel, which has several large empires. It's definitely not Scadrial, where all the population is clustered around one or the other of the poles.
Which planet in the cosmere has the largest population? Since we've seen the planets at different times, let's limit the question to periods we've seen in the books.