I have a group of log files (they are related but that's application level and not relevant). I want to set up logrotate configuration on Debian linux to rotate the whole group (all the files) once any of them reaches the limit, say 100MB for example. I can't see such an option - any idea how to achieve that?

bash: man logrotate.conf

File selection

   missingok
          If the log file is missing, go on to the next one without issuing an error message.  See also nomissingok.

   nomissingok
          If a log file does not exist, issue an error.  This is the default.

   ifempty
          Rotate the log file even if it is empty, overriding the notifempty option (ifempty is the default).

   notifempty
          Do not rotate the log if it is empty (this overrides the ifempty option).

   minage count
          Do not rotate logs which are less than <count> days old.

   maxage count
          Remove rotated logs older than <count> days.  The age is only checked if the logfile is to be rotated.  rotate -1 does not hinder removal.  The files are mailed to the configured address if maillast and mail are configured.

   minsize size
          Log files are rotated when they grow bigger than size bytes, but not before the additionally specified time interval (daily, weekly, monthly, or yearly).  The related size option is similar except that it is mutually exclusive with the time  interval  options,  and  it
          causes log files to be rotated without regard for the last rotation time, if specified after the time criteria (the last specified option takes the precedence).  When minsize is used, both the size and timestamp of a log file are considered.

   maxsize size
          Log  files are rotated when they grow bigger than size bytes even before the additionally specified time interval (daily, weekly, monthly, or yearly).  The related size option is similar except that it is mutually exclusive with the time interval options, and it causes
          log files to be rotated without regard for the last rotation time, if specified after the time criteria (the last specified option takes the precedence).  When maxsize is used, both the size and timestamp of a log file are considered.

Source 1: Debian Bullseye "man" command
Source 2: https://linux.die.net/man/8/logrotate

The above, does include a Single file, while also use for a complete Sub-Directory

Full Examples

Here you decice, what, a single file, a bunch of .log or / any

/var/log/folder/*.log {
    minsize 10M
    maxsize 50M
}

/var/log/folder/single.log {
    minsize 10M
    maxsize 50M
}

/var/log/folder/* {
    minsize 10M
    maxsize 50M
}

i think that will solve all your questions.