Best way to resize all items on Sketch artboards while maintaining same layout? [on hold]

I am fairly new to Sketch and I prototyped a re-design for a website. However, after I finished all my artboards I realized all my pictures and text are way too big... Is there a way to resize everything but keep everything more or less "in place"?

Also any tips on designing to the right proportion of a Webpage on Sketch? I feel like it is easier to visualize for mobile apps and the preview for this is also better on Sketch; for Webpage previews it doesn't show the true size unless I zoom in until the screen fits my laptop (which is a bit of a hassle to do constantly).

Thanks in advance!