The dehumidifier you need will depend on the moisture levels in your home and also the size of the room that you’re looking to keep the dehumidifier in (the place where moisture is prevalent). Factors like if you live in a humid climate, if lots of people spend time in that particular room, if there’s a washer/dryer machine near the room that will increase the size of the dehumidifier that you need.
Ultimately, even if your moisture problem is not too bad, getting a larger dehumidifier (bigger capacity) than you need is not a negative thing as it’ll still work effectively and you may be able to use it on a lower setting.