unfortunately the best option I can see is supporting Lineage OS development on the XZ2 Compact at the moment. Seems to be no market for smaller phones at all (Apple are even ditching the mini for 14 iirc, Jelly has never made it anywhere near me to try).
Beyond that, the Xperia Ace 3 is an okay size (slightly bigger than iPhone Mini) and maybe if it does well there will be more smaller options?
I've a 12 mini now because I thought the original SE was losing support last year, I def regret not sticking with the SE for another year instead, for me the 12 mini is unreasonably big.