Thank you very much for the screenshots and SVG documents. Turns out, I misunderstood your problem. Now that this is no longer the case, I suggest the following approach:
1. Import all the desired SVG documents.
2. For each imported symbol, open its context menu, choose "Properties", and specify the desired width and height.
(After step 2, all the previews should have the undesired small size.)
3. Open the palette section's context menu once again and choose "Convert to Document".
4. Open the palette section's context menu a final time and choose "Adopt from Document".
Repeat steps 3 and 4 for each palette section with resized templates.
Alternatively, you can quit and restart yEd once after resizing all symbols instead of repeating steps 3 and 4 for each palette section.
Hope this helps.