Your sup should be fine to learn on with a smallish wave sail, somewhere round 4m or there abouts. Although maybe something a little bigger will let you set it up on a 400 long mast, which'll be more adaptable with bigger sails later.
Ideally try and rent a more traditional learners board with a really small sail for the first few goes, if there is anywhere near you that rents equipment. The first few learning steps are so much easier on a ridiculously wide board with a handkerchief sail, it just lets you wrap your head around manoeuvring the board/sail with a bit less falling in. I wouldn't go out and buy a learner rig though, you'll soon want something bigger.
Summers almost here, have fun