A couple of years ago, when we explored apples and one of our Thursday Themes, we took a stab at figuring out the circumferences using string. You can also use string to figure out how tall your pumpkins are.
Have children hold string next to their pumpkins from the top of their stems to the bottom of the body. You can help cut the strings so they are about the right length of the height of the pumpkin. Now, you have several strings of different lengths that can be laid next to one another to create a graph all on their own.
If you use yarn, each child could have a different color so they know which one is theirs. Otherwise, you need to label the strings so the children can remember which one is theirs. These varying lengths of string can be used for comparisons, for sequencing and for documenting the different heights of the pumpkins.
Another way to measure the height of the pumpkins is by using Unifix cubes. The children can put the cubes together until they are the same height as their pumpkin. You can then count the cubes to see which pumpkin is the tallest and which pumpkin is the shortest.