RE: How to change SSRS report design to landscape orientation?
If you are on the most recent (Sept 2021) release of SSRS Report Builder, go to the view tab, and put a check in the Properties box.
The properties window will appear on the right side. It should read "Report" under properties. If it is showing one of the controls on the report, then click in the big empty gray area outside the body of your report so it'll show the properties of the report.
Now click on the right most icon right under report (looks like a white square). This will bring up the report properties.
On the page setup tab, you can easily change the paper orientation among a slew of other settings.
I know this is an old post, but hopefully it'll help others still looking for the answer.
ArcaneCode