Depends on how you want it to look
. I have some perle cotton in my "practice" basket but not sure what size it is; if you cut off 18-20", it's easy to separate into 4 strands, and 1 strand is plenty fat enough for big stitch work -- about the size of a heavyish crochet thread. I also have some 12wt cotton sewing thread, which could also be used for big stitch with a somewhat lighter, crisper finish; it's about the weight of 2 strands of embroidery floss. Any of those will work fine in a size 7 embroidery needle. I have a slimmer embroidery needle that they also work in, probably an 8 or a 9, nice big eye for threading.