Find the smallest file

Bash + coreutils, 13 bytes

ls -Sar|sed q

Explanation:

ls -Sar|sed q
ls            # list files
   -S         # sorted, biggest first
     a        # show hidden files
      r       # reversed (smallest first)
       |sed q # q is quit at first line that matches given regex, 
              # given regex is empty so guaranteed match.         

PowerShell, 30 24 21 bytes

(ls|sort le*)[0].Name

Try it online!

ls is an alias for Get-ChildItem. That's piped to sort-object with the length attribute, so the files are sorted by size. We index into that with the (...)[0] to get the first (i.e., smallest), and then take the .Name thereof. Output via implicit Write-Output happens at program completion.

Saved 6 bytes since we're guaranteed that only files exist in the directory. Saved an additional 3 thanks to ConnorLSW.

Python 2 3, 94 76 74 54 bytes

-18 bytes thanks to @orlp
-2 bytes thanks to @Jonathan Allan
-20 bytes thanks to a change in challenge specs

from os import*
print(min(listdir(),key=path.getsize))