Say that I have an image that I have cropped to a tiny size, e.g. 50x50 px, but when blown up on my rMBP screen it’s actually being rendered at something like 1500x1500, making scalable overlays and text look crisp. How can I save this file the way I see it, at 1500x1500, and not have Fiji squeeze all my scalable into 50x50 raster, besides screencapping everything?
Use Image > Adjust > Size… to upsample your image to the desired resolution, choosing the desired interpolation method. Make sure your image is displayed at 100% zoom, then you’ll see every pixel as it is saved.
Below are two cropped versions of the Blobs sample image with nearest-neighbor (None) and with bicubic interpolation: