Hi Suvas.Chandra ,
There's nothing as such as standard when it comes to dashboard .Its up to you how you want to present your story , your findings .
From here you can select the different paper size and mode i.e. landscape or Portrait .
Play with it and check which one suits you .One thing to keep in mind while creating dashboards keep the size same for all of them .
I know this. I was wondering if there is any size as the best practice someone is using.