How much is the zoom of Sun-Pluto eclipse image

The "raw" LORRI image is 1024x1024 pixels. The field of view is about 0.29°, see section 3.6 of this paper, or abstract of this paper.
Pluto is a little less than 250 pixels in the raw image, corresponding to 0.29° x 250/1024 = 4'15'', well consistent with your estimate.