So, if you still want to do this, and only if you're hosting your own server, press the ~ key (above the Tab key) on your keyboard, then type the following:
FindClientByName(name).Player.SetPlayerScale("# # #");
In that console command, replace "name" with the person's name whose size you are attempting to change, and replace the "# # #" with 3 numbers anywhere from 0 to 5, but no more than that. You can enter higher numbers, but it will just revert back to 5. Try messing around with that, for instance: "3 0 5" or "5 5 0". Be sure to put the spaces in between the numbers.