I think you will have to experiment a little to find the lowest size
that will protect the radio without blowing out from a little current
surge. Start with the lowest value that seems reasonable and see if it
blows. If it does, replace it with one of higher value until the radio
plays satisfactorily. I think that there is some debate as to whether a
transistor will pop before a fuse but you will have done all you can to
protect the circuit.
Other problems can come from spikes and surges in the power line or
lightning hits. You want to have a good surge protector in the ac line
and for further protection, you could add a "crowbar" circuit which
protects from overvoltage.
For 100% total security, you could run the radios from a gel cell or
sealed lead acid battery and use the Astron to charge up the battery
off-circuit. The battery also has the advantage of being free of line
noise.
A lot depends on how steady your power lines are and how often you
have lightning storms.