The largest RAM in a single address space every built must have been somewhere around 1 PB to 1.5 PB. Meaning a computer where one CPU core can address every byte of a 1PB address space, using load and store instructions, without going through network protocols. I don't know whether the current Summit/Sierra computers (which are about 3-4 PB of memory) have a single address space, but the slightly older IBM P775 did. Note that these are all NUMA architectures, where the memory performance varies greatly with the logical separation from the memory (like L1/L2/L3 cache versus memory, just extended out to remote memory).
In practice, these computers are typically configured with highly partitioned memory, each CPU only using memory that is relatively near it, for better performance.
For amusement: The 64 bit address space only allows for 16 EiB of memory, and we are today already at multiple PB, so we're getting within a factor of 1000 of running out. For disk storage, we have run out: there are file system where the capacity can no longer be measured in a 64-bit number.