A paragraph is a minimum of three sentences, from what I remember. So, really, you shouldn't be double-spacing unless you have at least three sentences. However, for short posts, I tend to just go "Meh" and doublespace all I want.
This letter, however, abuses this heartily. If you're writing a tl;dr, use PARAGRAPHS. Not a wall of text, not itty bitty sentence-by-sentence bites, PARAGRAPHS.
Also, I have consciously made this post at least three sentences per paragraph. I am a dragon. Pizza is delicious.