In this case we actually have physical Bibles from before Constantine's time. There wasn't much of a universal canon, but we have planty of fragments from the second century.

Marcion came well before Constantine, it's much more likely that the actual editing happened in his time. He in fact assembled the first canon, consisting of Luke and Paul (except 1 & 2 Timothy and Titus, forgeries which had yet to be written, and Hebrews which is mistakenly attributed to Paul). The books he had were themselves much shorter than they are now. The church fathers claimed, and to this day most still assume, that Marcion edited them to remove things that contradict his beliefs. But when more closely examined, it turns out that he probably didn't change anything and the church just kept adding more and more stuff to them over time, sometimes in response to Marcion himself. Romans in particular is much longer than it originally was, adding several whole chapters.