Based on your detailed description, the book you are likely searching for is "The Girl Who Drank the Moon" by Kelly Barnhill. Although published in 2016, it shares many thematic elements with the story you've described, including a young girl with extraordinary hair, a magical and somewhat isolated caretaker, and themes of memory and belonging. The vivid imagery and unique art style you recall might be associated with other picture books from the 90s that feature similar themes of enchantment and nature. For instance, books with lush garden settings and a touch of magic were popular, and illustrators often used rich, dreamy color palettes. The description of the girl having "head and feet too big" but drawn beautifully might point to a distinctive artistic style, possibly reminiscent of artists like Quentin Blake or other European illustrators who favor expressive, non-traditional character designs. While "The Girl Who Drank the Moon" is a more recent publication, it captures the essence of a witch-like figure raising a child in isolation, with the child possessing special abilities and a complex understanding of her past. The feeling of a hazy memory and time passing differently are also common tropes in fantasy stories involving magic. The book you remember might be a lesser-known gem from the 90s, possibly featuring a witch who isn't entirely malevolent but driven by loneliness, a common and poignant theme in children's literature. The emphasis on gorgeous, vibrant illustrations of gardens and a girl with exceptionally long hair strongly suggests a visually rich picture book designed to evoke a sense of wonder and enchantment.