Hi there Qubits.The GCW Zero has a resolution of 320x240 and a 3.5-inch screen, measured diagonally.
This means that the width is 71.12 mm, fitting 320 pixels, and the height is 53.34 mm, fitting 240 pixels.
A pixel on the GCW Zero is thus 0.222... mm by 0.222... mm. This is a real pixel, in all senses of the word: it displays one color across all of its surface area, using the red, green and blue subpixels to create the illusion of a certain color. It's not suddenly a fake pixel just because its dimensions are different.
The size of these pixels cannot be reconfigured, because the color filters that make up the red, green, and blue components cannot be physically moved. As such, it is impossible to resize one of the pixels on the GCW to be a different size (here, the GB/GBC's 0.2552 mm pixels).
It's also not possible to make a pixel display a gradient of colors from its top to its bottom, to emulate a larger pixel by lighting up one entire pixel of a certain color, plus 1/6 of the top, bottom, left or right of its neighbors, for example.
Your image states that the resolution of the GB/GBC screen on a GCW is 142 by 131. That is false. 160 by 144 pixels are being used to display the 160 by 144 pixels of the GB/GBC screen - the same happens with the 240 by 160 (not 213 by 146) pixels of the GBA.
What you want is for a 160 by 144 pixel screen to take up the same space
as on a GB/GBC. Because the GCW's pixels are 0.222 mm and this cannot be changed, you need 184.7887 pixels by 153.8931 (but GIMP says 166 preserves the aspect ratio better) pixels on the GCW.
Here's the title screen as you'd see it on the GB/GBC.
There are two ways to go about changing the pixels to maintain the size as opposed to the indivisibility of the pixels.
a) Duplicate or delete pixels, which makes text look bad (especially if it duplicates or deletes a horizontal or vertical stem).
b) Use bilinear interpolation, which is closest to the idea of "use 1 whole pixel of a certain color, but have the top, bottom, left and right 1/6 of its neighbors use the same color" but blurs the output.
The scalers on GCW emulators assume that you either don't want the blur or that you want the image to be full-screen, which offsets the illegibility that the blur creates. Here, though, the high amount of blur created by resizing the image by exactly 14.86% would outweigh the increase in legibility inherent in enlarging the image. It's a trade-off.There are images in this post. Note that, because I don't know the size of your monitor, obviously I can't make them display at the same size on your monitor as on your GB/GBC. Don't worry; those are still real pixels, they're just a different size. You may also display these on your GCW and see exactly what they'd look like and the size of the result, by uploading them to your GCW and using the image viewer application.