wish it were that easy to size. But the roles, logging setup and traffic volume on those systems is more important.
the only piece of advice I would give you at this stage is to consider separating ES and GL from the beginning. This can absolutely be run on a single node, but I’ve found it easier to expand the solution if ES and GL are run on separate systems from the get go.
if you want some real life data, my DIY graylog deployment is ingesting approximately 1.5 GB of logs per day, it’s single node with 16GB of RAM and 8core AMD fx cpu (basically a desktop PC from few years ago) aaand it doesn’t sweat much, graylog is limited to 3 cores (via docker) because it overheated cpu and it works flawlessly processing logs from our UTMs (HQ and few small remote locations) and servers (host linux server, windows domain servers etc), it’s deployed alongside a small TIG stack processing metrics from our servers