I agree that you should either measure by volume in your brewing vessel or look into purchasing an electronic scale. There are several different electronic scales you can purchase online that are fairly inexpensive. I tend to favor using a scale because sometimes it is hard to judge volume, especially with each tea having it's own particular shape and density.
If you are going to weigh your leaves with a scale, I would recommend experimenting with different leaf to water ratios in order to find out what suits your taste. I find that I end up using between 1 and 2 grams of leaf per ounce of water, depending on the type of oolong. I usually brew oolongs in a gaiwan gongfu style with these measurements.
Measuring by volume will also work fairly well if you become accustomed to filling the leaves up to a certain point in your brewing vessel. Regarding your question about the leaf expansion of balled up oolongs vs. long shaped leaves, here is my recommendation for measurement. For balled up oolongs such as Ali Shan, fill up your brewing vessel 1/4 to 1/3 full of leaves. For long, twisted oolongs such as Baozhong, I recommend filling your vessel 1/2 to 3/4 full of leaves. You can use more leaf with the long oolongs because they are less dense and are already slightly unravelled.
Whatever method you choose to use is just a matter of personal preference and also how much time you have. Each method will probably take some tweaking, but experimenting with different ratios is half the fun of it. Good luck!