We're somewhat limited by the available RAM on the hypervisors that host the instances.
The standard flavours of VM that are currently available (4 vCPU / 16Gb RAM; 8 vCPU / 32 Gb RAM) are designed to balance the CPU/RAM resources of the hypervisors, so that they nicely divide up into a whole number of instances, while still providing a relatively powerful machine.
An 8 vCPU / 256Gb RAM flavour would be unbalanced in favour of RAM and would mean we could fit less instances onto our computing resources, so we don't plan to offer this as a standard thing.
That said however, we do have extra capacity, outside of our regular hypervisors, for high-memory VMs. We are aiming to provide access to these on a time-limited, request basis.
You're the first requester, so we don't really have a system in place for this yet, but if you send me a PM we can work it out!