How should I modify the Newbie Routine if I want to go for mainly girth gains

For example should I use longer or shorter strokes?

First you need to finish the newbie routine. If you have, start jelqing at a higher erection level for starters.

At starting, you need to do both stretching an jelqing, because you have to learn how these exercise will work on you. When you’ll be more expert, you can focus mainly on jelqs, or dry-jelqs, or add some other excersises linke Uli’s, just as example. But for now, do it straight.