What is the best way to get exact pixel size from screenshot images made in Pages (v14.2)?

I do 99% of all my website image needs using just Pages, tweaking and layering images and backgrounds and then taking a screenshot of what I need (because I never learned how to do all the amazing stuff I can do in Pages inside the Photoshop Elements app I have). Periodically, a web designer will ask for images in exact pixel sizes, which I don't think Pages can do. Right now, to do that, I will save my Pages-made screenshots, then open them in PS Elements, and adjust the size from there. Is there anyone else that does this, or has a better way to get exact pixel size from Pages? Hope this makes sense.