You really need to heed the advice about looking to the manufacturer of the voltage stabilizer to determine the rating of the fuse. The reason is that the stabilizer is limited by both an overvoltage constraint and an overcurrent current constraint. As long as the maximum voltage at the receptacle in your home is less than the maximum voltage rating on the stabilizer nameplate, you don't need to worry about the overvoltage limit.
But the overcurrent limit is different. As some others have noted, there are a different kinds of electrical loads. Another way of parsing these loads is into those that are constant impedance (such as lamps) and those that are constant power (such as motors). There are also constant current loads, but none of the components in your enlarger system will have that characteristic.
The fan wants to absorb constant mechanical power (measured in kw in Europe, or horsepower here in the US where our political leaders force us to use an archaic system of measurements). Constant mechanical power into the fan means constant electrical power into its motor. But as the voltage drops (for the reasons we now understand), the current drawn by the motor will increase - electrical power is equal to the product of voltage and current. But if that current flows through a device that is temperature-limited, the concern is that a reduction in voltage could result in excess thermal stress on the voltage stabilizer.
The manufacturer should specify a fuse rating based on his knowlege of the thermal withstand capabilities of the components in the stabilizer. That knowlege should include both the maximum current and the duration - and hence the manufacturer should specify both a current rating (in amperes) and a time rating (slow, fast or something more elaborate) for the fuse.
The point you will need to be aware of is that it is possible that the fuse that you have is the appropriate fuse, and the fact that it is blowing could be telling you a critical bit of information - that the rreduced voltage from the power supplier is below the minimum voltage that the stabilizer is rated to withstand and deliver rated power output. Actually, that relates to one of the questions you asked - is it possible to use a smaller lamp in the enlarger. Switching to a smaller lamp would eliminate the fuse blowing problem, but it would also affect the printing time with your enlarger.
The analogy that I like to use is to think of that distribution circuit as a rod that you are holding in your hand. If you wiggle your hand a little, the end of the rod at your hand moves up and down a little - but the far end of the rod moves a lot more. That's exactly what is happening on that distribution circuit - as the voltage varies a little back at the utility source, the variation out at the remote end (where you live) is a lot more. The ideal solution for you is one that is dynamic and that can maintain a constant supply voltage to your enlarger (or any other appliance in your home) as the input voltage from the utility varies over the entire range. If the range of variation is too large, it may be too expensive to try to solve the problem for a small load like your enlarger.
The problem you are experiencing with your enlarger is affecting every other appliance in your home, and also all electrical appliances owned by your neighbors. Perhaps a better solution is to revisit the notion that the utility could do something to improve voltage regulation on the circuit.
The explanation from your power supplier that because you are in a rural area, they cannot boost the voltage to you without causing an overvoltage at other customers is bogus. I suspect they are being lazy and trying to avoid having to solve the problem. There are several possible solutions they could apply - some as simple as locating power factor capacitors at various points along the distribution feeder, while others include installing an automatic voltage regulator part-way down the feeder to boost the voltage for those customers further away from their source while leaving the voltage closer to the source unchanged. Other solutions include distribution series capacitors and various forms of static compensation devices.
In the US, distribution utilities are required to satisfy performance standards that include (among other things) the tolerable range of voltage delivered to the consumer. If there is an equivalent regulatory body in Spain, they may be able to help you (and your neighbors) put some pressure on the utilty supplier to correct his problem. As a minimum, you should determine if the low voltage you have measured is considered an acceptable delivery voltage under local standards.