Question for the large pieces

So I want to make some big pieces, and a friend showed me blue marble. I'm not sure how to make the images smaller but keep the details. I guess I don't know how to word this, my bad yall (I don't have Photoshop), and it's also hard to see what I'm doing whenever I put an image into Blue Marble. If y'all have any tips or something that'll help, I appreciate it.