A lot of big dick guys like me know what we are packing and if you find a guy who cares about the bottoms he tops, then he’ll likely readily admit that his cock is not a “starter cock”, i.e. not ideal for virgins.
I definitely know mine isn’t a starter cock, yet I have taken a few virgin holes. Doing so required a lot of prep through foreplay and the bottoms often prepping beforehand such as with a dildo. As mentioned in thread already, dildo prep is the best prep you can do. If you truly want to prep for a larger dick, then you’ll want to have a dildo with a 6” girth at minimum, length is good at 7” but that is really up to you on how long you want your prep dildo to be.
The difficult part in any anal sex is fitting a dick’s girth. Length can be an issue but is typically less so, because you can simply not insert the entire length. I’ve definitely gone a few times topping where I’ve only inserted 3/4 of my length and not the whole thing. Girth, however, is an all or nothing though. Sadly I’ve been declined for my girth a good many times too, either from the start or after much effort trying to insert my dick and it just being too big for penetration. Foreplay can help, but is limited in how much it can help, because foreplay is in the moment and sometimes much more time is needed for the bottom to relax. So that’s why I really advocate for bottoms to self-prep before ever seeking to hookup with big dick guy.