Commenting out the line — which shouldn’t even be there, because systemd already mounts a tmpfs on top of /tmp even without the entry in /etc/fstab — does not change the maximum size of one’s /tmp. The default is always going to be half of one’s RAM.
tmpfs does however allow one to specify its maximum size via the mount options — see…
man tmpfs
If you want /tmp to have a maximum size of — just as an example — 6 GiB, then you would put something like the following in /etc/fstab. 
tmpfs /tmp tmpfs defaults,noatime,mode=1777,size=6g 0 0
You can also specify a percentage of your RAM instead. So if e.g. you want to reserve 40% of your RAM, you would do something like this…
tmpfs /tmp tmpfs defaults,noatime,mode=1777,size=40% 0 0