Not an Apple fan but their SE gets updates for a good amount of time, about 1cm bigger length and width than their first SE but it might fit your criteria.

Samsung Flip is a compact clamshell that becomes a regular sized phone.

The ASUS Zenphone 10 is on the smaller size but barely qualifies as a small phone so up to you, expecting updates to 2027.

The fairphone is not getting updates.

They are months behind, and many phones they should be updating to this day run extremely EOF software