5" seems to be the smallest common screen size. Tracfone (and their subsidiary brands Straight Talk, Simple Mobile, Total Wireless, Net10 Wireless) has several 5" phones for $25-40 right now.
Tracfone TCL A1 4G LTE Prepaid Cell Phone $25
Tracfone ZTE Blade T2 Lite 4G LTE Prepaid Cell Phone $30
Tracfone LG Rebel 4 4G LTE Prepaid Cell Phone $30
Total Wireless Samsung Galaxy J3 Orbit 4G LTE Prepaid Cell Phone $40
All run Android 8. I'd watch for sales or codes.
Of course, if you
really want a tiny screen, the revived
Palm has a
3.3" screen. It also runs Android 8.1. It's pricey, though.