Most of cases can be solved both ways, both bottom and top down. It's really interesting to do it both ways!
That is precisely why I recenty created the following case, with a detailed market sizing problem, in case it helps > https://www.preplounge.com/en/management-consulting-cases/candidate-led-usual-style/intermediate/market-sizing-new-startup-launch-baby-strollers-in-the-us-244
Hope it helps! Cheers,