Depending on what handset you choose, you'll be presented with a few storage size options. The majority of choices will start at 128GB of storage these days but some do drop as low as 64GB.
If you tend to save things on the cloud and rarely download any music, videos or general large files, 64GB will suffice. However, for most people, a minimum of 128GB will be a necessity.
The bigger question is whether its worth considering the 256 or 512GB options we're seeing these days, and even sometimes a 1TB option! In most cases, it's not worth it.
Unless you're downloading an entire library of Kindle books, hoarding Spotify playlists and trying to have the entirety of Netflix at your fingertips, these larger options just aren't worth the additional costs.
Of course, the easiest way to know is to look at how much storage you're using on your existing phone and work off of that.
Many Android devices offer expandable storage as well so if you've gone for a storage size too small, you can always remedy it with certain mobile phone deals.