The Fenway bullpens are 21 feet across. MLB recommends that bullpens be 27 feet wide, though it is not a strict rule*.
* - What is strict and unambiguous, however, is Rule 1.04(a): "Any Playing Field constructed by a professional club after June 1, 1958, shall provide a minimum distance of 325 feet from home base to the nearest fence, stand or other obstruction on the right and left field foul lines, and a minimum distance of 400 feet to the center field fence." At least seven parks are in clear violation: Yankee Stadium and parks used by the Orioles, Rays, Astros, Pirates, Giants, and Padres. MLB has done nothing about it.
Larry Lucchino, on the pens:
They're among the narrowest in baseball, if not the narrowest. It makes it hard for two guys to warm-up. It does have other effects in terms of reducing the depths of right field, which is among the deepest in baseball.The proposal must be approved by the Massachusetts Preservation Society.
According to Ballparks.com, the distance to just right of center field, into the triangle, used to be 550 or 593 feet in the early 1930s (though I think this was the distance to the back wall of the old center field bleachers, where a batted ball could have conceivably ricocheted.) The distance to the right field foul pole has changed many times, going from 313.5 feet in 1921 to 358.5 (1926), down to 325 (1931), back up to 358 (1933), then down to its current distance of 302 (1942).
Theo Epstein says the bullpen is his #1 priority this winter. "We never had that third reliever step up ... [but] the history of high-paid, non-closing relievers with long contracts is not pretty". Epstein added that it would be a "disservice" to think that "injuries were the only reason" the Red Sox failed to make the playoffs.