So, with the way that the Arenas are set up, there's coding in one of the misc files (either 01 or 04, I don't remember offhand) that tells the game which Titantron Size/Style to load -- this will determine if they use the in-game Raw Style Titantrons (0), Smackdown Sized Titantrons (1) or Older/Classic sized Titantrons (2)
Outside of the Raw V4 arena by Rev that had the pre-thunderdome Raw Set which they changed up the display for (they merged and displayed the tron/mini/floor stuff together and changed the dimensions of what was needed), all of the arenas use the same general tron sizes so you won't need to worry about different sizes/dimensions.
Also, for if all 3 sizes are required...yes and no.
There's a file called assetconv_p30.pac that tells the game various things about the superstars, including which Trons they are allowed to use. You'll notice that many of the Legends/Classic wrestlers in the game only have one titantron in the WWE 2k19/Movies/Titantron folder (usually the classic, _2 size). The Assetconv file also tells the game that this is the only titantron they have/will use, and so their video will play in any arena, albiet at the 640x640 size, which may not take up the full size of the titantron depending on the arena (I believe Revoniko made it so that their current day Raw and Smackdown make the legend/classic style videos fullscreen in an update to the arenas, previously it was just a smaller video in the middle). In this case, it's okay for the wrestlers to only have one titantron, and they don't need the other two ones.
Now, most wrestlers and modded slots will have all 3 tron types (0, 1, and 2) enabled for them. If, on the off chance, the Assetconv file tells the game to load a tron that they don't have for the arena, (i.e., they only have a 2 but the arena uses 0 or 1), the video will not load properly, usually as a mirror image on the tron or a black screen. This will rarely happen, as if you use CCT to import one tron, it will fill up the other slots with the same tron as well (i.e. if you have a tron that you import into the 0 slot, it will add copies of the tron for 1 and 2 as well). Also, you don't always have to adhere to the dimensions, as the game will usually attempt to resize/display the trons in a proper fashion. (So, as an example, if you import a 640x640 video into all three slots, it won't ruin anything)
Sorry for writing a whole bunch, but I hope this helps/sheds some light on how the game loads trons!