You do sit higher up, same as a 29er, despite the 26" rims the tires are that high you are soon at the same level as hybrids and 29ers....
My first fatbike was a 19" size L mukluk, was perfect, so wanted a 18" pugsley to be around same size but was advised to go for the 20" as surlys are strange to size.
Mines a 20" but that's to the top of the little triangle on the top tube, BB to centre of top tube is 18"....I had to get in touch with shop when my pugsley came, as I thought they'd send me a 18" but box and shop said, 20"
It's perfect