I think that below 45 - 50 feet an "engine room" is difficult to justify from a space utilization point of view... Some voluminous center cockpit designs do pull it off, though.
If you're looking at a boat under that size then I think that in this area, of prime importance is the amount of access that's provided to the engine. In our case it's in a box under the companionway and we have 100% access on 3 sides, with arm's reach on the fourth. All in all it's a workable arrangement, though it's tight for any significant soundproofing measures.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)