Both 512px & 256px PNGs convert to 128px only and am unsure about how to force other sizes, specifically 256px

[mutogene]

[FoxP]

Hello,
Default included resolutions in the converted ICO files are 256x256, 128x128, 96x96, 64x64, 48x48, 32x32, 24x24, and 16x16 pixels.
So a converted 512x512 PNG file will have a maximum layer of 256x256px.
You can check with Gimp:

Source (512x512):

Result (max 256x256):