What resolution(s) should photos be to be shown on iPhone?

I have an app where users have profile photos.

Profile photos will be shown frequently, and are about 3/4 the size of a regular iPhone.

Of course, the tradeoff is bigger resolution means higher quality but also more data and higher cost.

I am building a pipeline to generate thumbnails of appropriate sizes after they are uploaded.

What sizes should they be for a good user experience?