Are you asking about a sheet of frozen fish food or cubes? I buy the big sheets of Mysid. I take the whole sheet out of the freezer, place it on a board (like a 1 x 4), let about 3/4" - 1" hang off the edge, and just push down. It snaps right off. I keep doing that, and then take those strips and break them off. I end up with various size pieces, but most of them are about 3/4" x 3/4". I put all of that into 2 separate Freezer zip-lock bags. I also add Garlic and this is a good time to add it, one time, to all the food.
By-the-way, I'm no chef but I know it is not a good idea to freeze, thaw, freeze, thaw food.