Just downloaded faststone out of interest – not the first time it's been recommended on here.
At first glance / use, it looks pretty good but doesn't solve the portrait / landscape issue. If you want all your pics to be 800 pixels (say) on their longest side, there's a checkbox in Photoshop's Image Processor which will make portraits and landscapes 800 pixels on vtheir largest side.
Can't find a similar thing in faststone – am I missing it?
I think you're missing it. Go to resize, check the box/tab which says "resize based on one side", pick the side you want, pick the size you want. Job done.