What is the maximum size of an allowed .HEX file to be download to a Cyclone V deice?
Moreover, in case I want to use a few .HEX files, there is a required guard addresses between each of them?
.hex files are used to initialize on-chip memory blocks, so you would, at most, have one .hex for each memory instance in your design. They would be the size of the memory instance they are assigned to. So the "maximum size" is the size of the associated memory instance.
I don't understand your second question since each .hex is separate from any other.