Due to the size constraints, there’s not much to recommend (I am currently typing this on an iphone 7 lols). Since you mentioned GrapheneOS, which can only be installed on Pixel devices, I’d recommend the Pixel 7a. It’s definitely bigger than an iPhone 7, but it is the newest Pixel device. To get a phone that’s smaller that supports Graphene, I’d have to recommend a Pixel 5a instead. It’s older but closer to the size you wish for. Pick your poison.