The cheap ones will/can vent directly out the back into the wall (~ 3 1/2"x10" duct - size of a heat register). Most other will vent up through the top wher you can vent through the wall (make a 90 degree turn) or up through the roof.

Hoods are based on the size type cooking appliance BELOW the hood and the requirements for venting and cooking style of the owner of the appliance and make up air and the height of the ceiling vault and the construction of your home. Impossible to answer your question with the information provided. Where is your kitchen designer, GC and/or builder.?