MTB sizes (S, M, L) don't translate very well in to road bike 'mm' sizes. That, and the geometry is way different, especially 'modern' ('00s and later) MTBs.
The DB Century is a pretty relaxed road bike, even when compared to an Endurance bike like the Cannondale Synapse, or a racer like the Specialized Allez.
However, at 6-0 with a 32 inseam, i'd think a 54cm would be pretty small. I'm 5-10 with a 30, and I typically ride 56's. (and a vintage 57) 54's are too small front-to-back, regardless of how high the seat is.