Do try! And don't worry, lots of people do cosplays of ayu during her concerts. I see at least 10 every time I attend a show.
If you wanna be recognizable, choose some of ayu's quirkiest outfits, like the polka dot pink Ladies Night from AT2006, the pink maid outfit posted by Lacus, this tour's bunny suit, or the sparkly rollin' outfit from AT2009 complete with bob haircut.
If you don't wanna stand out too much, try something from her magazine shoots/TA photos. Those resemble normal clothes more, and you won't feel so conspicuous wearing it. And don't worry about not being recognizable, mostly everyone there is an ayu fan, they will recognize everything lol!! I've seen concert-goers wearing:
And don't worry about costume size and everything. I saw a girl in
Microphone PV dress during one of the Yokohama shows. Yeah, the black one with the huge skirt. It was so awesome I tell you, I was so proud of her for making and wearing it! ^^