I would think twice the size of yours could easily be had, but a problem you may run into is that there are very few butcher shops that break down whole beef quarters anymore. It's all done at corporate processing plants now and shipped out as "box beef" or, like Walmart gets, pre-cut and packaged meats. So you're stuck with whatever the store has, no specialty cuts.
There really isn't much true "shoulder roast" in that area. Get a little above and behind there where the shoulder blade is and it's chuck roast, or blade roast or 7 bone chuck, like this...
View attachment 654567
The upper bone in this picture is the cow's shoulder blade, or 7 bone as it's called. Some meat cutters or butcher shops may fudge it a little depending on what the customers want and what they can charge, but the muscle groups start to change some. But I wouldn't hesitate to do a boneless chuck roast the same way you did the shoulder. A meat department that actually cuts meat will cut you one about any size you want...