I use a free app called ResizeIt - AFAIK it’s available for both Mac and Windows. Point it at a folder, tell it a max pizel size (horizontal or vertical) or a percentage, and hit Go. The resized images appear in a new folder.
I use this to do what the OP wants.
Yes, I found out about this from a woman who maintained a community group’s photo-intensive website. Same reason, same need.
I like Format Factory. It is free, and can do entire folders at once. It’s main job is converting from one format to another like .bmp to .gif and reverse, or .mov to .mp3 and reverse, and many others. I’m pretty sure it can change the resolution, and can certainly convert them to smaller files.