You can configure the memory via dremio-env but as far as cores go we use what is available. We can throttle that also via dremio-env using below, where 8 is the number of cores per executor
-Ddremio.debug.sysopt.dremio.sliced.num_threads=8
As far as how much memory and cores you require, that entirely depends on your workloads, type of queries, type of joins, how wide are the tables, joins etc?