The fuse size should be marked on the label, on the circuit board, or on the holder. All fuses are (or at least should be) marked, but it is sometimes hard to read. What type of fuse is this? A tubular glass one? If so, look carefully for a stamped value on one end.
The label may also indicate the size.
The last thing you want to do is install a fuse bigger than the thing needs, otherwise you are liable to burn something up if it hasn't already happened.
Note, it could be that it really needs some other repair, and a fuse won't help.
Fuses can fail for reasons other than an overload, but that's usually the case.