Straps should be placed fore and aft of the keel in most cases. Sometimes, due to prop shafts and other components it's not possible but that's usually with bigger boats. I lifted a 19' boat with two chain hoists and a couple of large straps. because I didn't use any beam or bars to attach the straps to, it squeezed the boat sideways and I ended up with spider cracks on the interior floor.
To measure the length of the strap, take a piece of string, line, etc, and wrap it around the boat as if it was a strap, then measure.
Boat stands would be best but if that's not an option, at the very least lower the boat on to something to steady it while working on it. A dangling boat over one's head is a bit daunting, especially when the shoal keel could act as a guillotine!