We hear from some players who like to play certain sized puzzles, but not others, and want to purchase volumes of only those sizes.
Unfortunately there's no way currently to purchase specific sized puzzles. The bundles are priced assuming that people will play about half the puzzles (e.g. just easy or just hard ones), so they include 100 puzzles of various sizes as described – identical in distribution to the 100 free puzzles at the beginning.
The alternative to this is the (USD) $4.99 monthly subscription which gives access to 10,000 puzzles of all sizes. Many people do this if they play lots of puzzles and prefer specific sizes.
Sorry the specific size you're looking for isn't available separately. We did think long and hard about this, but the app gets complicated quickly for everyone if there's a big menu of sizes and quantities, so we decided to keep it simple and just throw in more puzzles instead and kept the price the same.