Check with your longarmer - if she says 5", she probably means 5" on ALL sides. I've never had one smaller than 4" on all sides, and usually they want more in the range of 6".
A (hopefully helpful) word of advice - tell your longarmer that you are new, and ask her to explain the things you don't know or understand completely. If she does so graciously and patiently, then use her services. If she gets cranky about having to explain things to you, run far, far away. Good communication is essential for a successful experience for both you AND her, and good longarmers who want your repeat business and referrals will take the time to make sure you understand everything.