It's a romantic comedy. And it stars ''Alias'' hottie Jennifer Garner as a 13-year-old girl. Actually, she's 13 on the INSIDE but grown-up on the outside, and...maybe it's better if Garner explains. ''It's like a female 'Big','' says the actress of her first romantic comedy, in which her character, Jenna, wishes for adulthood, then wakes up one morning a 30-year-old New York City magazine editor. But, adds Garner, ''she's plopped into her [adult] life with amnesia, and has to figure out what's going on and why everyone's treating her differently.'' Including her childhood best friend (Mark Ruffalo), a nerd-turned-foxy photographer to whom Jenna turns for help. Think ''Memento'' in Manolos. Director Gary Winick (''Tadpole'') predicts ''Alias'' fans will be delighted to see Garner's funny side in his $37 million big-studio debut. ''She's a comic genius,'' he says, ''right up there with Lucille Ball.'' Though chances are Desi never hollered at Lucy, ''Stop being a spy!'' as Winick did with his star. ''I don't even realize that I don't walk normally anymore,'' laughs Garner. ''I catwalk down a hallway, ready to pounce.''