Considering this printer is a Chinese designed and built printer, the most obvious choice for screws is metric system screws. These are most probably M2 screws; if you look at the screw heads (Allen key screws) the size with respect to the fan (40x40x10 mm => fan thickness is 10 mm) seem confirm the assumption (head height is 2 mm as seen in the image of the dimensional analysis below, which is typical for M2 Allen bolts).
The length should be easy to calculate if you have a caliper laying around, else you need some ingenuity using a ruler and an object smaller than the diameter of the screw and a maker (to measure the threaded part in the metal cover). Measure the lug thickness of the fan (about 3 mm if you look at the image of the dimensional analysis below), add some length where the screw enters the metal casing (about 2 mm). For the bottom screws you'll need slightly larger screws since the shroud is also attached. Two sets of 2 screws should do the trick, you'll need at least M2x5 and M2x8, but measure before you order. A reference for the Ender 3 (which seem to use the same hot end casing) refers to M2x8 and M2x12 screws:
The stock blower fan uses something like M2x8 for the top, and M2x12 for the bottom. The hot-end cooling fan is M3x10, and so are the screws that hold the fan shroud on. The heat-sink screws are M3x14.
Dimensional analysis based on image: