I think he’s asking how to make a small test print of a portion of an image that will be an exact match to the resolution/detail size on the final full size print.
With a little math this can be done.
1) Final goal is (say) a 30x40 print. Want to print an appx 8x10 " section at same resolution.
2) Determine pixel width of full image. Call with W1.
3) Crop desired section and determine its pixel width. Call this W2 and your ratio is W2/W1.
4) Print the crop at a print width of W3 = W2/W1 times 40 (the width of the final print).