If all the image have the same aspect ratio that will be very easy use Menu File>Automate>Batch. You need to create an action to do the resize use the Photoshop Plug-In Script Fit image. When you record the single step action to use the Fit Image Dialog the set the size tou want the images to fit in. Set Both the width and height to the size you want the longest side to be. Fit image will constrain the resize so the images aspect ratio will not be chabges so Portrait and Landscape images can be processed in a single run. The Size you set in the Dialog will be recorded into the action step, This size will be set into the script whan the action is played the Scripts dialog will not be displayed. All image will be resized to fit. Only if all image had the same aspect ratio like 3:2 or 2:3 will the resized image all be the same size.
If Images do not have the same aspect ratio you will need to crop all image into the same aspect ratio before doing the Fit Image to have all output images have the same size.
Check the Image Processor script: File - Scripts- Image Processor.
If it doesn't quite get you there, check out the Image Processor Pro script that Russell Brown provides online.