Buy a few different weights of Nielsen slugs and a few different weights of Varmint Knockers and try them all to see what shoots the best with your gun. After the initial go-round, fine tune your gun to the particular slug that shoots the best. Nielsen 33.5, 36.2 and 38.5 as well as the Varmint Knocker 31.5, 32.8 and 34 all work well in my guns. It would be a good starting point and give you a fair range of slugs to start with for initial testing. It's not usually like as with pellets where you pop in a JSB or H&N and it just shoots great from the get go. Slugs almost always require some sort of tuning with either the hammer spring or regulator to optimize performance. Some guns shoot a particular slug at 840fps well, while others might like that same slug at 930fps and still other guns might not shoot it well at all. Take your time, take notes, write down all of your settings, be patient and have fun!
Stoti