The starting amps for the M53 are 25.9. You would really need to know the starting amp rating for your ice box to get an accurate number to size your generator. The 6.5 amps you mention are the running amps. The compressors on the icebox always require more amperage or power to start because of the extra torque needed. Once you can obtain that number, multiply it by the voltage (120), add 3108 (the starting watts for the M53 pump) and add 20% to the total. That would tell you exactly what size generator you would need to run both appliances at the same time. Amps times volts give you watts.
I can ASSUME that the icebox would be in the neighborhood of 32 amps. At that rate, together with the sump pump running at the same time, I would estimate that you would need between a 6000 and 7000 watt rated generator. If you can get away with switching between the two appliances, you could go with a 4000-5000 watt rated generator.