There are limits to what can be reasonably stored within a String variable but if you really want to store a large piece of formatted text within one then I suggest you use HTML line-break br elements and paragraph p elements to format the text.
<<set $var to "<p>This is the text of the first paragraph.</p>">>\
<<set $var to $var + "<p>This is the text of the second paragraph, it contains a line-break<br>within the text although technically that does make it no longer a paragraph!</p>">>\
a. The above is not a recommendation on the best way to store large pieces of text within a variable, it's just an example of how to include HTML elements within such text.
b. One of the main points of allowing a story to consist of multiple Passages is so you don't need to store large pieces of text within variables, so doing the above defeats this purpose.