Welcome
I have a friend who built that model several years ago, but did not put sails on. He no longer has instructions nor sails.
I built a different lateen rigged Heller kit, the le France. I made my own sails, as I wanted to display with sails furled. In that case I made the sails as long as yards, but much shallower (depth perpendicular to yard). My reasoning was that cloth unlikely to be anywhere sailcloth thickness in scale, and I was afraid if I used actual sail dimensions the furled sail would look too bulky. I feel this worked well, so you might consider that route.