Partially untrue
.NET 8 works on macOS and Linux; the issue you are having is that Windows Forms .exe does not work for those operating systems unless you use a compatibility shim like Wine, because the .exe uses Windows Forms (Windows-specific drawing GUI stuff) rather than something like Avalonia.
The repo is divided into multiple projects. PKHeX.Core being just a code library, no GUI, which works on any OS, and PKHeX.WinForms that uses Windows Forms.
The releases that are posted are not code-signed, and new releases are "new" in that it is new to the antimalware scanning (not a scanned definition). The release pipeline is me just clicking Publish on the WinForms csproj and zipping the exe; nothing special beyond that.
If you have an extremely high DPI monitor, then yes it might look tiny. If your OS is sandboxing it, then it might be interfering with its rendering. Without seeing a screenshot, can't diagnose/advise.
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There have been discussions about Linux/Mac support, and the answer is always that you're free to port whatever. The GUI code is some of the oldest & least abstract code in the repo, and most of the contributions are in the PKHeX.Core side of things as we try to keep up with mechanics on each new game content release. The GUI has been a relative afterthought, and porting >100 forms/controls to another GUI framework when the current one works well enough for most users.
Using Avalonia would require each release to include the Avalonia GUI libraries (or require people to install something NOT from Microsoft), which would be a "bloated" exe rather than the ~22 MB zip it is currently. MAUI hasn't shaped up well (Linux support for Desktop lol). So the suggestion from us has always been to just use Windows/vm/wine on a desktop PC, because nobody wants to spend the time porting it to another GUI/mobile framework with full support.
Running as a web app (wasm blazor) has its own pitfalls, like some crypto (md5, aes) not being supported (so can't export certain save files).
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Checking the contributor graph (and knowing the content of every commit), PKHeX is mostly a solo-contributor project with others chipping in occasionally. I have other projects and interests, and duplicating the GUI (learning a new framework/language) is not something I have much of an interest in.