Space between paragraphs is an alternative to a first-line indent for signaling the start of a new paragraph.
The worst way to put space between paragraphs is to insert an extra carriage return.
As with first-line indents, you want the space to be large enough to be easily noticed, but not so large that the paragraphs seem disconnected. Four to ten points of space will usually suffice. The larger the point size, the more space you’ll need between paragraphs to make a visible difference.