Surely longer swipes bring better leads. With short swipes people just click the link in front of them and maybe opt in. But people who click at the bottom of a longer email are more invested in what you have to say. It might be harder for sellers to provide the clicks but lead quality for the buyer might be better.
There needs to be a balance I guess.
People these days have short attention spans and long emails have to be well written and captivating enough to get them reading to the end.
Also, the length of the email depends on the type of product and purpose of your swipe. For lead capture, the shorter the better.