You'll have to tailor your purchase to your daughter's interests and abilities.
Since 50 pieces is too easy for her, I would go ahead and try a 100+ piece puzzle.
Looking at different products, I see that 50-150 pieces are typically for ages 4+, 5+, or 6+. 100 piece puzzles look to be suitable for 5+ and 6+.
However, 200 piece puzzles often jump to the range of 7+ or 8+, so that might be too big of a jump for her.
Whichever size you get, I would introduce the new puzzle with a comment such as,
"You did so well putting together the last puzzle, I thought you might like to try one more challenging."
Then, if it turns out to be a little too difficult for her, you can say,
"I'm very impressed with how hard you're working on this puzzle. It sure looks like fun. Do you mind if I work with you?"
By introducing yourself into this activity you can make it a fun, bonding experience.
As a note, some children are going to be better at puzzles than others. Jigsaw puzzle solving requires a different set of mental skills. Spatial recognition is needed for for matching both the piece shapes to each other and the piece art to the larger image. Memory skills are needed for remembering what the whole picture looks like when they're not looking at the box. Strategy/critical thinking skills are needed to come up with solutions to solve difficult portions (such as doing borders first, grouping pieces by the same color, etc). If your daughter excels in these areas, she might be equipped to handle puzzles outside the listed age ranges.
However, part of the reason the larger piece-count puzzles are for older children is that the pieces are smaller. Thus, they require a little more dexterity (fine motor skills) to handle. So, you'll also have to pay attention to whether or not the size of the individual pieces is affecting her enjoyment of the puzzle. These size differences are why I was able to find a 100 piece puzzle for ages 3+, and another 100 piece puzzle for ages 5+. The 3+ puzzle had much larger pieces.