We keep young royals in 9litre rubs to start with two hides water bowl with half the rub on a heatmat connected to a mat stat. I also provide some plastic leaf vines to give more cover.
Increase the size as they grow to eventually a 50 or 64 litre rub (no difference in base size just height)
I tend to use an 18 litre as an inbetween. If you wanted to go up to a viv or even start with one then you can work your way up in size. Best way to make them secure is to give them lots of places to hide.
I dont have any experience of putting a young royal straight into a viv so I cant comment on this, others will have more experience will be along and give you their stand on this.
Good luck with your new Royal