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.
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.