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.
".. 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)
1984 Fast/Nicholson 345 "FastForward"