Bulk scaling photos on Export

Hi all,

I am fairly new to all things Adobe and not technical-minded, so forgive me if this is a basic one.

I use Photoshop to watermark a large number of photos and am sure there has to be a more straightforward way to do this. Majority of my photos are in the 20MB+ range when I import to Photoshop. I then need to watermark each one (side question, any way to bulk watermark every photo?? I duplicate the watermark layer to each photo, but again this is fiddly and time consuming. If i could apply the same layer to every file i have open in one go that would solve it, but haven't worked out if that is possible?) back to my main issue, once all photos are watermarked, I then 'Export As...' and then fiddle with the scale until i get down to below 4MB (should have mentioned, our website only takes files 4MB or less so every photo needs to be reduced). This is extremely hit and miss and time consuming and I am sure there has to be a better way to export all at a set size..?? Just can't find anything with my days of Googling.. appreciate any help, thank you very much.