How do I increase size of per-user tmpfs partition /run/user/$(id)?

Fresh installation of Ubuntu 16.04.

File /etc/fstab contains no references to /run/user/...

mount prints

...
tmpfs on /run/user/115 type tmpfs (rw,nosuid,nodev,relatime,size=1612068k,mode=700,uid=115,gid=124)
tmpfs on /run/user/1002 type tmpfs (rw,nosuid,nodev,relatime,size=1612068k,mode=700,uid=1002,gid=1002)
tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,size=1612068k,mode=700,uid=1000,gid=1000)

I need a couple gigabytes more in /run/user/1002.

Where do I tune it?

You can increase it by setting RuntimeDirectorySize in /etc/systemd/logind.conf as mentioned here: https://wiki.archlinux.org/index.php/Profile-sync-daemon#I_need_more_memory_to_accommodate_my_profile.2Fprofiles_in_.2Frun.2Fuser.2Fxxxx._How_can_I_allocate_more.3F

The standard way of controlling the size of /run/user is the RuntimeDirectorySize directive in /etc/systemd/logind.conf (see the man page for logind.conf for more). By default, 10% of physical memory is used but one can increase it safely. Remember that tmpfs only consumes what is actually used; the number specified here is just a maximum allowed.

From the logind.conf man page:

RuntimeDirectorySize= Sets the size limit on the $XDG_RUNTIME_DIR runtime directory for each user who logs in. Takes a size in bytes, optionally suffixed with the usual K, G, M, and T suffixes, to the base 1024 (IEC). Alternatively, a numerical percentage suffixed by "%" may be specified, which sets the size limit relative to the amount of physical RAM. Defaults to 10%. Note that this size is a safety limit only. As each runtime directory is a tmpfs file system, it will only consume as much memory as is needed.

You will need to log out and in again for the changes to take effect.