The smallest practical programs I'm aware of are RP2040 PIO, which are max 64 bytes each (32 instructions of 2 bytes per instruction). Various 8-bit controller

The smallest practical programs I'm aware of are RP2040 PIO, which are max 64 bytes each (32 instructions of 2 bytes per instruction). Various 8-bit controllers give you 256 Bytes to 128kB Flash / program space. Various 32-bit uCs, including ARM Cortex M0+, ARM Cortex M33, ARM Cortex M4, ARM Cortex M7 give you spaces from the 2kB to 1MB sizes. There are entire worlds and chip environments between the smallest 64 bytes programs and the 256kB programs around.