Which ICO sizes do you actually need?
The largest image is not the only useful one. Build a size set for a specific destination, then inspect it at actual scale. A larger source cannot guarantee readable details at 16 pixels.
Start with the Windows size set
For a Windows-oriented file, Microsoft recommends including 16, 24, 32, 48 and 256px. In IcoJet, 24px must be selected explicitly; 64 and 128px are also available. These are pixel dimensions, not print DPI settings. IcoJet does not offer every size an application framework may request.
Separate automatic resizing from hand-tuned artwork
IcoJet derives all selected sizes from one image. If a small drawing needs thicker strokes or fewer details, revise the source or use an icon editor that lets you assign different artwork to each size. Selecting more sizes does not perform that design work.
Verify the exported container
Use an ICO-aware viewer to inspect its embedded images; a viewer showing one preview does not establish that the others are missing. Try the real destination at more than one display scale. If a legacy program rejects the file, check its encoding requirements: this tool writes PNG-compressed entries and does not offer a legacy BMP-encoded ICO mode.