if you know the output voltage and current of this psu, or the total current ...
you can summarize a value like this : from ohms law
ex: 24vdc at 2 amps is 48 watts
48 watts / 120vac would give 400 ma fuse, you could play safe with an 500ma
sadly the fuse is the last thing to blow in a smps ...
do you have writtings on the psu casing 100w is ok but do you have voltage somewhere ? if you wrote 220 to 120 vac
it is : 100 / 120vac = 833 ma ?? an 1 amp would be okay ? i think unless you have other problems