iOS: Green strip to return to numeric keypad much too small

Tapping the green colored status bar on iPhone takes you to the current call screen.

If you are finding it harder to tap it because of the smaller size of the hit-target, you can achieve the same result by an easier alternative approach. Open the iOS app switcher UI by either double clicking the Home button (for iPhones with Home button) or swiping up from the bottom (for iPhone without Home button). (Refer to the Apple support document, Switch apps on your iPhone, iPad, or iPod touch)

Now swipe left to access the front-most item in the app switcher UI. The frontmost app would correspond to the current call UI. You can tap on it to the same effect as tapping on the green color filled status bar shown at the top when a phone call is in progress.

Note that this approach works for a call made with either the Phone app (Cellular call) or Apple's built-in FaceTime app. For 3rd party apps, you'd have to tap on the corresponding app in the app switching UI.