You need imagemagick for this.
In Debian/Ubuntu just run this command as superuser:

apt-get install imagemagick

Linux and Mac

Just use this script:

#!/bin/bash
I=0
for f in *.jpg
do
(( I++ ))
r=$(printf "%03d" $I)
 read w h <<< $(convert "$f" -ping -format "%w %h" info: )
 if [ $w -gt $h ]
 then
  echo "$f is $h tall and $w wide (landscape)"
   convert "$f[800x>]" $r.jpg
 else
  echo "$f is $h tall and $w wide (portrait)"
  convert "$f[395x>]" $r.jpg
 fi
done

, where 800x> is settings to resize, more about them here.

Most used are:

• 800x> = if width more than 800px then shrink width to 800px with original scale.
• x800> = if height more than 800px then shrink height to 800px with original scale.

Microsoft Windows

You can use same imagemagick command, but foreach file iteration from Windows must be used.
Universal command to resize all images in folder:

convert '*.jpg[x600>]' %03d.jpg