Perhaps the last time you rendered your scene, you had the output resolution set to 320x240 and rendered a BMP file. Re-rendering the scene using different output settings is fast and efficient with command-line rendering.
“c:\program files\autodesk\[program folder name]\3dsmaxcmd” -outputName:“c:\program files\autodesk\[program folder name]\renderoutput\myImage.jpg” -w 800 -h 600 “c:\program files\autodesk\[program folder name]\scenes\myscene.max”
and press Enter.