When people were taught about that event, would they have been taught to have visualized all of them on the same side of the table, that big of a room, etc? Or was it painted with specific details that were requested?
Edit: Also, what would the customs of been for eating at a meal like that one when it was originally held?
Da Vinci's The Last Supper is a special portrayal of the biblical event because it was commissioned by Ludovico Sforza, the Duke of Milan, to be placed in the dining hall of the Santa Maria delle Grazie monastery, which the Duke had recently had renovated. The reason this makes it special is because Sforza requested that the scene be painted as if the monks, whom would be eating beneath this painting daily, were dining with Jesus and his apostles, which is why they are all portrayed to be sitting on the same side of the table.
In real life, this would be a very awkward way to set a dining table, as it would be hard to make conversation with those at the far end of the tables. In portraying the dinner in this awkward way, Da Vinci was able to make the monks feel as if they were truly dining with Jesus and his apostles, as well as easily paint every member of the dinner with such striking detail that, even after the painting fell to ruin in ten years due to a failed experiment by Leonardo, who, instead of painting in the fresco method of painting on wet plaster, painted directly onto the wall, the details were still sharp and discernible, allowing them to survive to this day.
How this differs from what people at the time would have visualized the last supper as being, of course depends on the person. The bible provides little detail as to the setting of the supper, and rather focuses on the dialogue and intrigue of the dinner. However, as we can see by other representations of the event, there are many different artistic portrayals of this famous biblical scene. While we will never know how this looked exactly, any of these works can be a guess as to how it actually looked.
Sources:
/u/explicit_snark already answered your question, but I wanted to provide you with this link:
You can search for "The Last Supper" here to see how different artists depicted it. "Same side of the table" is actually fairly common.
