s, m, or l.
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 ongoing billing is by observed usage, a bigger size or higher ceiling
does not reserve CPU, memory, or disk; it just allows your workload to spend
more. The one-time creation charge varies by size as shown below. 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.