I've been trying to work out how to make default replacement covers for the novels in the "Write Novel" dialog. It wasn't too difficult to figure out the how (straightforward Replace in SimPE), but the What has proved frustrating. Nowhere, but nowhere, is it stated what size the replacement image is supposed to be. No templates, no measurements, no nothing.

Is the actual size less important than the proportions? (And what are the proportions?)

Depending on which program you're using, there are likely various methods of doing this.

As an example, in Photoshop you can find the size of the area by marking it, copying the marked area, and making a new document/file.
Make sure you copy an area that's fully colored, as copying any transparent parts will get you a different size than the marked-out area.
This will copy the size of the marked area into the new document, so you can check out the size (before opening up the new document, and under "Image->Image size").

There's also an info button, but it only seems to show the size in cm (or possibly inches if you use that).

I'm not sure if this method works in other programs, but it's possible there are other similar ways of figuring out the same issue.

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Make sure the texture itself uses the same sizes (usually ...256/512/1024...) and has the same proportions (1-1, 1-2, 2-1), as the original. Sometimes you can size up or down in increments, using the "power of 2" rule, but it depends what the item you're working on is, and how useful this is going to be for how it shows up ingame. If it's for UI items, it's often best to stick with the original size/proportions, at least on first try.

If there aren't any other defaults you can download and check out, you'll want to extract the textures from ingame and see how those are built up (think of those as your templates). Is there extra room on the sides that doesn't appear ingame? Probably not a good idea to put anything there. Sometimes you have to just test and see what works.

Paintshop Pro will tell you pixels if you know how to ask it. But it seems what I needed was a modifiable image in the first place, which couldn't be extracted from the defaults OR from the replacement I was trying to work with. (I got something, but it was not recognizable or usable.)

To cut it short, Slig's replacement covers did provide the info I was looking for (and a tip that the cover wraps around inside-out and backward, too!). They're sized at 256 x 256 but with a strip below the cover images for "text inside" (same as with Epi's diary replacements). If I wanted more detail, would 512 x 512 work as well. as long as I kept the proportions correct?

Probably would work fine, but keep in mind how the items show up ingame (size on screen - especially for UI items, and for items how close up you'll ever need to get in gameplay), and if it's worth it to make the size of all the textures 4 times bigger. Some replacement textures don't need to be (or don't benefit much from being) that big. Bigger textures are more memory-consuming than smaller ones (sometimes more so than high-poly objects).

If it's an object (not sure if you mean the actual diaries, or just the ones showing in the UI?) you can extract an UVmap to get the exact area for where to put the design. That's often very helpful. I tend to use a 3D program while texturing, so I can see a 3D model with an applied texture without having to open the game all the time (SimPE isn't always an option).

There were a few more wrinkles to this than I thought. The wraparound book covers with inside text are not the same as the cover images that show up in the "Write Novel" dialog. Those are even a tad smaller, for who-knows-what reason, and are not reversed.

I think I've got it solved now, though.