In general bigger is better with antennas. But you need to know direction, distance, UHF/VHF stations. Sometimes you need two antennas. Attics often give 50% less signal than outside due to shingles/ roofing felt. I give Denny's antenna service my info and they will help you select what you need. I should of put up their link earlier, here it is.
http://dennysantennaservice.com/