Hmm...I'll throw in my two cents here. I'm a girl with short hair (really, as in doesn't-touch-my-ear short) and while I sometimes wish I had long hair to play with and braid, ultimately I am more comfortable with short hair. I can wear hairbands or bows if I want to look extra adorable, but otherwise I am rocking the tomboy look. It sometimes clashes with extremely feminine outfits, but it's something I have learned to ignore, the same way that I ignore that I am obviously not in the body of a toddler. So as others have said, it's more about your attitude than the length of your hair.
That being said, in my eyes there's nothing cuter than a little boy or little girl with a head of curls! They don't have to be super long, but maybe long enough to form some ringlets? Or like lopearedboy said, just let the top grow out a bit.
I don't know if you usually use product in your hair (gel or wax or the like), but most short hairstyles look immediately more feminine if you don't use any product and maybe let some strands fall into your forehead. That will also invite people to play with it!
I don't know if being a boy sissy means you want to be a boy that's forced into dresses and playing with dollhouses or just a soft boi who maybe likes pink and thinks dresses are cute and comfy but also is a bit of a rascal with scraped knees and swordfights, but either way, I think both can be achieved with short hair and some suspension of disbelief and a dose of imagination.