Which mounting screws to use for Ikea Hoppvals?
There is much info  on how to mount blinds on the internet.

This type of blind USUALLY  mount inside the window casing ,to the top of the window casing which is typically wood. If so then 1"  wood screws would be sufficient to deal with the weight of of the blinds and the mechanical forces of pulling them down. 

BUT the photo on the IKEA site seams to indicate that the bracket is mounted to the wall, If this is how you will be mounting them ( and your wall is made of drywall ) then you you need to know if there is structure in the wall at the mounting location. If there is structure, stud or header etc,  then any good wood screw ( at least 1 1/4" ) will work. If not you will need a toggle anchor to deal with the forces involved. 

I prefer Toggler anchors. 

As manassehkatz says, you need to know what your wall is made of in order to know how to proceed.