Hi Shaun,
the minimum requirements really depend on your application. Just for an example, real time paparazzi for rotorcraft needs around 130kbytes of RAM and around 200kbytes of flash (that's how big the bin file is) for STM32F4xx chip (without much optimization). CPU load is only a few percent.
Standard rotorcraft code for STM32F105 (lisa M 2) takes also around 200kb of flash (bin file), and probably around 20-30kb of RAM (I don't remember exactly). CPU load is typically around 50% but can go up to 70-80% with certain applications (FPU control loops).
Indeed there is room for code optimization, but this should give you at least a rough idea.
Lisa M uses STM32F105RCT6 which is not too different from your chip, so if you are careful with code size, it should work.