The process API gives you maximum 2.000 x 2.000 px output (might be 2.500x2.500, not sure). So I am guessing that your BBOX, when “measured” in px of 10m size, is larger than that.
I suggest you try with smaller area.
For larger areas we have another service:
https://docs.sentinel-hub.com/api/latest/#/BATCH_API/batch_processor