How to gain confidence with new "big" bike?
Not to be trite, but: Get out there and ride!

I just tried out a 29 Plus mountain bike a few months ago, first time on a MTB in years. Felt really weird and disorienting at first, so I just rode on smooth gravel pathways for a while. Once I started feeling a bit more confident in it, I started going out of my way to run over small rocks, twigs, branches, etc., to get a feel for how the bike responds. Quickly built up to "cinder-block" size rocks (breeze-blocks if you're in the UK). It was really fun "relearning" - it's the first time in a long I've felt like a little kid on a bike. 

There's no rush, nobody's going to appear out of the bushes to laugh at you when you decide to go around a larger obstacle that you're not yet confident rolling over. 

It's not clear from your post what your level of experience is, but if you're feeling really nervous and cautious, just ride on a pavement for a few days. Try rolling off curbs (check for cars first!). Try rolling along on some grass. 

Have fun with it!