The Manchester United striker was named as England’s permanent captain by Roy Hodgson earlier this week.
“Rooney looks a bit chubby,” Forren told TV2.no.
“They’re obviously good players, and many of them are on good form already after three games in the Premier League but we’re looking forward to it.”
Forren was signed by Southampton in January 2013 but failed to make a single first-team appearance before retuning Molde the following July.
England host Norway in their only warm-up fixture before the start of their Euro 2016 qualification campaign which begins in Switzerland on September 8.
"It's almost a bit scary but it's in games like this and experiences like this you want to be able to contribute so I'm looking forward to it," Forren said.
"We know we're a good team so there's no reason for us to walk around thinking we can't stop them."