How big is big for a women's labias?

Only you know. Labia come in all shapes and sizes and have different coloring. There really is no "too big" or abnormal so if you're bothered by the appearance or if they are bothersome in swimsuits or exercise, then see a specialist for an evaluation as genital plastic surgery can improve appearance and function dramatically.
Variable. It is just the way she developed. Everyone is different so large to one woman may be normal for another.
Labia size. There is no true answer. Labia minora can extend beyond the labia majora many inches. Each side may also be different lengths. On some women, the labia minora cannot be seen at all beyond the labia majora All are normal variants. .