I am wondering how big and how difficult it will be to find our flixbus to Siena in the Tiburtina station. I’ve read somewhere that this bus station is huge or confusing or something. Any clarity on what the station is like would help and also how early to arrive before departure.
Have you tried Googling “Tiburtina bus station map” ?
There is a Tiburtina train station. That is large. The bus station of the same name (just next to the train station) is much smaller. It should not be hard to find your bus there. The tickets for the bus generally suggest being at the station 15 minutes early.
The only thing that confused us was that the exit door from the Tiburtina metro station that we went out of was below the level on which the Flixbuses are --- so you don't leave the metro and just see all the buses in front of you. A map doesn't show you this difference in elevation, but you have to go up some stairs and cross a street. Perhaps a different metro exit door would have been less confusing, I don't know about that. The stalls for the various buses are all lined up and are well marked. But do leave yourself plenty of time to find the right bus and don't be afraid to ask people if you are doing the right thing.
Visual cue: Flixbuses are all a screaming lime green with a giant Flixbus logo in white on both sides. Very hard to miss! Yes, the Tiburtina bus station is just out front of the main entrance/exit to the train station but across a couple lanes of traffic. I think you won't have any trouble but do allow some extra time. There are electronic sign boards at each bus bay telling you the next bus and its destination. This is more helpful if you know the end destination, just as for trains, because sometimes the intermediate destinations aren't as visible on the signboard.