Here is an illustration of the 20" Skyelander 650b with an estimated saddle height of 617mm based on a height of 5' 1". As you can see this size Skyelander is likely to be too big for you assuming an inside leg measurement of 699cm (27.5") - the standover height of the bike will be circa 6cm higher than your inside leg so it might be worth considering bespoke geometry of this frame based on your specific measurements - we would recommend a sloping top tube (angle tailored to your inside leg measurement), a greater bottom bracket drop and a shorter top tube. Please email us on email@example.com to discuss your custom Skyelander.
To measure your inside leg: 1. Get a book and stand with your back against the wall, 2. Place the book in between your legs as high as it will go, 3. Move away from the wall keeping the book against the wall and finally, 4. measure from the top of the book to the floor. It will also be useful to measure the saddle height (bottom bracket centre to the top of the saddle) on your current bike and compare it against the illustrations.
We look forward to designing a bespoke Skyelander 650b for you.