What size nipples are your LOs using if they're bottle fed?

Mine is on #2s (Phillips Advent Natural bottles). I'm thinking about trying #3s. The last week or so, she has started taking forever to finish a bottle. I can't tell if it's just that she's not hungry or if it's not fast enough for her anymore. It started around the time she started rolling all over. Sometimes she acts like she's bored of eating and starts looking over at her toys, like she's trying to figure out how to get to them. But every time I offer the bottle, she opens her mouth. She even starts trying to feed herself. So I'm not entirely sure what to make of it but thought I might try a bigger nipple.