I couldn't find any reason why they picked 53 bits. This design grew out of the NBS's SEAC and DYSEAC computers which had 45 bit words, so it's unclear where 53

I couldn't find any reason why they picked 53 bits. This design grew out of the NBS's SEAC and DYSEAC computers which had 45 bit words, so it's unclear where 53 came from. SEAC only had fixed-point operations so I think it's coincidental that 53 bits matches a double. The interesting thing about DYSEAC is that it was "portable", weighing 20 tons and housed in a truck for the Army.