The amount of pork isn't critical. Another half-pound or pound more or less isn't really a big deal - if the bigger packages are a better deal, go for it. You could easily make 5 pounds of pork, as long as it fits in your slow cooker.
This is a two-day recipe, but there's not much work involved. You could cook the pork the first time overnight, refrigerate it in the morning, and finish for dinner.
You could also opt to cook this in a Dutch oven on the stove top or in the oven - whatever works for you. The one key here is knowing your equipment. The slow cooker I used for this holds the moisture in really well, so I didn't have to worry about the liquid evaporating out. With different equipment, you might need to add more water during cooking.
Slow Cooker Barbecue Pork Sandwiches
1 bottle barbecue sauce (about 15 ounces, plus more as needed)
1 cup water
If your slow cooker has a browning setting, brown the pork on all sides. If your slow cooker doesn't have a browning setting, you can do the browning is a separate pan, Or, just cook without the browning. You'll get enough flavor from the sauce, so you might not notice the difference.
Add the barbecue sauce (use your favorite) to the pork in the slow cooker. Cover and cook on low until the pork is fork-tender, about 8 hours.
Put the pork and all the liquid into a storage container and refrigerate until well-chilled. It's true that braised meats are better the next day, and it's not about time - it's about chilling the meat.
The other advantage to the chilling is that the fat rises to the top of the liquid so you can scrape it off and get rid of it.
Pull the meat out and shred it or slice it (your choice). Put it in a pot or saute pan, add all the liquid, and cook until the liquid is mostly evaporated and you have the meat swimming in a thick sauce. Taste for seasoning, and if you want more barbecue sauce flavor, just add additional sauce and let it simmer down some more.
Serve on buns. Pass more sauce at the table, if desired.