Fusible links aren't usually rated in amps*, because they're not intended to blow at a certain current. They're just intended to be a deliberate weak point in the wiring loom, the first piece of wire to melt if the circuit is overloaded (saves replacing the wiring harness). They're rated in cross sectional area, like the wiring loom wires.
The four fusible links by the battery are usually 3x 0.5mm2 (green) and 1x 0.85mm2 (red). In American wire gauge 20gauge is 0.5mm2, but 0.85mm2 would be 17.5gauge which doesn't exist. 17 gauge (1mm2) would probably be ok, since it's still smaller than the 3mm2 wires it's protecting.
LOCK: power door locks, power windows.
DEF: rear windscreen defroster, rear blower.
LIGHT: Take a wild guess. That's right; headlamps, interior lamps, stop lamps, indicators...
IGN: Anything that's switched on by the ignition switch.
Fusible link wire colours are standardised, but as usual the best thing about standards are that there are so many to choose from. Japanese fusible link colours are different to US colours. Yay!
The important thing about fusible link wire is that the insulation is rated to contain the melted metal without catching on fire. Regular wire of the correct gauge would melt at the same time as a fusible link, but it might also set fire to the engine bay.
*You can calculate how many amps it'll take to melt a fusible link wire, you just need to know the resistance of the wire, its melting point, ambient temperature, and thermal conductivity of the insulation. You'll end up with a graph that says "at 40A, this wire will take 10 minutes to melt. At 60A, it'll take 3 minutes (etc. and for example)). Regular fuses also work this way a little (a 60A fuse will usually handle 80A for a few seconds, longer if it's cold out) but they're designed to minimise this "feature".
