What size fuse for battery shutoff trickle wire?

I have a battery shutoff switch with a fused wire, so I don't lose my radio settings every time I switch off the battery. I have a 20A fuse in there, but it blew (which happens when I turn on the ignition and forget to turn on the negative battery switch). Of course, I have a bunch of buss fuses, but no 20A. I know I can just disconnect the fused link and drive it around. I have 15As. In a pinch, can I use that until I get the 20A? Or can I just use a 15A and be done with it?

Yes go with the smaller one. 15A