The magic number is 4.
Add 4 to your wire size and that is the correct fusible link size.
Example: A #10 wire will have a #14 fusible link.
The confusion lies in wire sizes. Some seem to think #10 to #14 is a drop of 2 wire sizes and that gives you the "2" number. It's really 4 sizes, but it gets confused because when you look at NAPA and the only thing between #10 and #14 is #12. The smaller, odd number wire sizes (like #11 and #13) aren't manufactured in primary type wiring (or at least easily available), but that is the wire spec that all this was/is engineered to.
A couple links of some reading.
Catalog
Fusible Link Wire FAQs
If you don't want to think about the sizes or it still confuses... Just add 4 to the number and there you are.
Cue someone saying "There is no odd size wire" and I submit this...
https://www.solaris-shop.com/content/American Wire Gauge Conductor Size Table.pdf
Here's some #1 wire readily available:
1 Gauge Marine Grade Tinned Cable - 100 Feet
Lot's of times, wire like this enameled magnet wire uses the odd AWG sizes, like this. Same specs for diameter, just different insulator or material. Point being, still using the AWG wire sizes as a standard.
