Biggest is the Sasha Grey Large and the Sexy Spades Large (both about the same size, the Spades is a bit wider but shorter).
Sasha I can put in as a first-of-a-session toy if I take enough time (though it's much nicer with some warm-up play), the Sexy Spades could probably be a first-of-a-session toy, but I've always used it second. Make sure you warm them up if your butt isn't warmed up.
They're both 2+", so I did need to move up to them. I started with fingers, moved to 1.5" sized objects, then up.