I need to work on this a little more, so I'm closing this temporarily.

Just mark it as draft

Sorry for the delay. I'm waiting when these changes will be merged the aniyomi fork. Because if my code would need corrections, I wouldn't want to bother both repositories to fixup them.

Added a size slider for the List Display Mode. It uses the same size variable as Grids do to determine size. However, the size of the List gets interpreted not as of columns per row, but rather as of entries of the list visible on the screen at once.

I don't really like this approach

Well... could you go into more detail as to why you don't like it? The way I see it, the grids scale as a number per row, so imho that's the logical way to implement this for the list.
If I went for a hardcoded size for all slider steps, several issues would emerge:
1) might look bad on some devices,
2) may needs further work if slider sees changes.

I guess in an ideal world the list would have to be rewritten as a one column grid to be able to utilize this Adaptive Cell feature (I hope I'm correct, I'm quite new to jetpack compose), but I don't need all lists to be affected, just the library and browse ones; nor does anyone have the time to do that.

What approach do you like? I'll see what I can propose. I still want to able to scale the list, that's why I'm here.

I don't have any ideas at the moment but one thing I can say is that this should be a separate setting and you get to decrease the number of visible items not increase it.