Fuse size

[tsubaki]

Look on the label of the device, there is usually a power consumption rating there. Or look in the book that came with the device or look on the net, most products have instruction manuals listed.

Find out what watt bulb you are using.

If needed convert Amps to Watts.
Amps x Volts = Watts
Watts divided by Volts = Amps

Total these numbers, add 20 % to this = minimum fuse size.

Best guess, 2 amp. Unless the wire feeding the devices is long or undersized for that fuse size. Then you will need to derate the fuse size accordingly.
It is easy to create a heating cable and not blow a fuse by using too small of a wire combined with too much length of wire and too large of a fuse.

Most cheap digital volt meters that read AC/DC also read up to 10 amps DC by connecting in line the meter with the load (bacically temporarly taking the place of the fuse), this will give you the current draw taking place at the time.
Problematically this only gives the reading of what is actually being consumed at that time, not what the device is capable of.