If you're gonna buy a rash guard look at surf companies. Rash guards were created to stop you from getting a rash while riding a surf board. They have many more styles and selections than any fight company and the quality is usually much higher, while being much cheaper
I have a RCVA rashguard, designed to be long. It even has a loop at the bottom of the shirt which I’ve guessed might be for somehow securing it to your shorts (putting drawstring of shorts through?)
If you hang dry your rash guard upside down then it will stretch the torso out and make it longer. I discovered this unintentionally but two of mine eventually stretched way out.