It's important to cut the ingredients into the specified size as potatoes, parsnips and rutabagas all have different cooking times. If you cut the vegetables into the right shapes/sizes, each will cook at the right pace so that all of the vegetables will relatively have the same consistency.
Also, I find the variance in vegetable size at the market to be extremely high; rutabagas and russets especially. I am going to give you the recipe using approximate sizes/weights, but I am also going to give you the scaling. If you don't have a scale at home to do this, have your intended quantities for each vegetable written down before you leave the house. When you get to the market, use the scales provided find your desired amount of the vegetables. You don't have to get it on the dot. This is very round about so being off by a few ounces or even half a pound wont hurt you. Remember, I am only using scaling just to remove some of the ambiguity in the recipe caused by the variance in the size of these vegetables; this is not a requirement.
Root Vegetable Mash:
Index = 5.25lbs = 100%
2 large russet potatoes (2 pounds) 46%
2 medium rutabagas (1.75 pounds) 33%
4 or 5 medium parsnips (1 pound) 20%
Olive oil or butter to taste
1. Get a large pot of boiling water going.
2. While the water heats, peel all of the vegetables. Cube the potatoes into 2 inch pieces and the parsnips into 1 inch pieces.
3. Slice the rutabagas in half and cut into 1/4 inch slices.
4. When the water is boiling, add the vegetables and cook until tender, 20 to 40 minutes depending on your water temperature.
5. Drain the cooked vegetables and return to the same pan. Mash and mix the vegetables with your desired amount of oil or butter. Season with salt and pepper to taste and serve.