It's very simple: Do the approach the is the easiest for you given the question.
Fundamentally, you need to take the approach that just makes the most sense in that circumstance. Quickly think about the key assumptions / numbers required and whether you 1) Know them or 2) Can reasonably estimate them. If you can, go ahead!
Here is a good Q&A great market sizing question asking to estimate # of electric charging stations in a city in 10 years:
This one could be answered top-down (as I did) by estimating population of the city, # of drivers/ cars, etc. etc.
OR, it could be answered bottom-up by estimating # of stations you see per block (or # of gas/petrol tanks), % increase this might be over time (or # of EV stations that would be needed per gas tank given EV stations take 10 times as long), and # of blocks you'd estimate the city to have.
Here's another market sizing (with answers + explanations) to get you started: