s or m.
Each size sets the vCPU count plus a default memory and disk ceiling:
Pass
memory_gib or disk_gib at create time to set your memory or
disk ceiling, up to the max allowable ceiling.
Because billing is by observed usage, a bigger size or higher ceiling does not
on its own increase your cost; it just allows your workload to spend more. The
default size is m. Choose s when you want the fastest cold starts, forks,
and resumes. Its lower ceilings also cap what a runaway workload can consume,
so you don’t accidentally use more than you need.