I chose a 10 amp fuse since that's what the original circuit called for.
Since I'm having trouble locating my ammeter, I did the next best thing. I inserted a 5 amp fuse and tried to start the tractor. It started twice, but on the third try the fuse blew. I attribute this to a slightly lower battery voltage after the third glow plug cycle. And of course as volts go down, current goes up. I then put in a 7.5 amp fuse. I started the tractor a number of times with no problem. Therefore the current under normal conditions must be more than 5 amps but less than 7.5 amps. I'll leave the 7.5 amp fuse in for a while and see what happens. I think a 10 amp fuse is about right. If the tractor sits for a while and the battery isn't topped up the 7.5 amp fuse may blow when you try to start it.
I think the fuse needs to be less than 30 amps to protect the relay. A 25 amp fuse may be acceptable, but a 10 amp is more conservative.