; well, at least this works in tasm 2.01 :)

 .model tiny ; this model is less than 64k for data and code
 .386
 .data
 .code
 org 100h ; this does wonders.. something about .com beeing loaded at
          ; offset 100h. Not sure, but include it :)

START:

  ; make your code of wonders here

  mov ah,4ch ; You don't want this in your
  int 21h    ; os, int 21h doesnt exist at bootup :)
END START

; on the commandline, do:
; tasm combase
; tlink combase -t
; ....and you will have combase.com, at the extreme size of 5 bytes!