I found
this tool (PicSizer) which resize images in batch mode, and does it very quickly.
best of all, you can tell it to only scale images down, so that if you have a smaller image it won't stretch it up (you wouldn't want/need to do that).
It's freeware, and OMG it is so fast.
I just ran it through all my collection and had it resize all my folder.jpg files down to 355x355 pixels (for those that where larger than that).