I'm 5'4" and can't ride in anything smaller than 17 inch. 17.5 inch is ideal. Height or weight doesn't actually have that much to do with it, it's the length of your thigh from hip to knee. I measured 17 inches last time I checked. A heavier rider will naturally be in a bigger saddle size than their thigh measurement might suggest, because otherwise their butt might not fit in the saddle, but MY butt fits in a 16 inch saddle and I can squeeze into a pony pad! Can't ride effectively though, because my legs are just too long.
If you're still growing add maybe half an inch to an inch to what fits you now, but add too much and your saddle will be too big, making it harder for you to ride well. For example, I ride quite well in my 17.5 inch dressage saddle, but used to have an 18 inch dressage saddle that I just could not ride in AT ALL. Half an inch really can make that much difference, depending on the saddle in question.
Oh and I actually DO ride in a saddle that's an inch too small for me - for jumping! It's a 16.5 inch all purpose with a deep seat and with my stirrups at LOW jumping length, I find I can't move properly in the saddle and have to ride in half-seat at canter, or else bounce a little bit on his back. I don't like doing that! Monty takes it as a cue to gallop, and there is no way I'm riding in half-seat for half an hour to an hour. So my saddle severely limits what I can do with my horse, because I can't shorten my stirrups to proper jumping length.
This coming from someone who rides with REALLY long stirrups normally!