It varies with the airline and the route. I can usually find a 757 or larger when I really want one. It's also good to look for internationally configured aircraft being repositioned as domestic flights. But if you have a specific airline or route that can make it tough. The airlines have discovered the efficiency of flying the smaller long haul aircraft (late series 737 and A320/21 crafts). The seat cost is lower and they are easier to fill. It can be funny to watch a heavy loaded one trying to gain altitude.

The LAX departure for example calls for a Westerly departure Iver the Pacific for 5 miles then circle back over LAX headed east but if you watch the flight path departing LAX you will see many of those smaller jets cannot gain the requisite altitude to fly over LAX when coming back so they usually return south of LAX. On the Western departure they are often still gaining altitude long after a 757 would have been in cruise.