There is 448KBytes on Cortex-M4 side. Usually, as M4 side does not embed complex protocol stack or display/graphic (which are devoted to be handled by Linux), there is enough room for complex 'coprocessor' usages. apart optimization, one way to reduce code/data requirements is to move more processing up to Linux side (whenever possible). Maybe look at https://wiki.st.com/stm32mpu/wiki/How_to_exchange_large_data_buffers_with_the_coprocessor_-_principle
System wise, DDR is not accessible from M4 side as this will end-up to complex Linux memory management issue as well as performance penalty on both M4 and Linux side (due to non-optimized transfers from M4 as there is no cache nor accelerator).
For some extended data storage, there is option to add an external SRAM on FMC (this choice is exclusive of Linux NAND usage on FMC). Performance is not best, but might work, depending on your application requirements.
Regards
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