Hi Mike,
You are right - we don't provide a way to edit postgresql.conf directly, and max_wal_size is not one of parameters we expose through flags. It is also unlikely that we will start exposing it in near future.
However, we set it to 1.5G on most of the instances. We did some preliminary testing and found out that on our infrastructure higher values provide diminishing returns while increasing crash recovery times. So we felt 1.5G is a good trade-off between performance and MTTR in case of a failure.
We may of course revisit this decision in the future. Please
open a feature request if you feel like this is limiting performance for your workload.
Thank you for using our service!