Here's how I suggest doing it:
Calculate the maximum wattage that your amp will consume. For example:
Heaters: 6.3V @ 5A = 31.5W (I picked 5A at random, your actual tube count might well use less)
B+: Voltage on first filter cap * (power tubes max signal current + preamp current) Say for a 6K6 amp: 350V * 0.1A = 35W
Total wattage: 66.5W
Now divide by your nominal line voltage: 66.5/120 = 0.554A estimated line current.
Now double it for luck - so in this case you would use a 1 amp fuse. The mains fuse is normally slow-blow too, to help it cope with the switch-on surge of cold heaters, empty filter caps and possible saturation of the transformer itself.