1-In Library mode highlight the thumbnails you wish to resize.
2-Click on the Export button which will bring up the export dialogue box.
3- Set the folder you wish to export the resized images to in my case it is a folder called Web
4- Pretty well follow my setting here and adjust them to your own preference.
When done click the add box and save it as a preset called Web so you do not have to keep setting it up.
All that remains then is to click the export button and let LR do it's work.
Shout if you need any more help John.