About first time

[Jeffry]

How big dick I will can handle for my first time?

[njjohn]

I would say the first thing shouldn't be a dick. Explore/expand your boundaries prior with something like this.
https://www.extremerestraints.com/rectal-dilator-training-kit.html
I would also suggest you find a lube you like maybe with a 2% lidocaine like I use and apply it yourself getting as much up inside as you can.