Depends on what you want to spend and how you want to use it. If you plan to move it around get a smaller one. If in one place get the biggest aperture you you can for your budget.
got my Lady a 130 i think it is, she likes it and it keeps her outa my hair too just got the big battery thingy for her Birthday pressy and the software for the laptop... Lots more free time for the bike next spring me thinks