Merged images popping in at all different sizes

I’m learning how to merge data and images into an InDesign file. I’m new to this process. I have a bunch of charts (PDFs) that I’m importing via the merge function. They are merging, but they are all over the place in terms of size (98%, 107%, 160%). How can I “force” them to pop in at 100%? 

And secondly, is there a way to make sure they always merge in at the same x/y coordinate? 

Thanks!