I could tell you the way that Italians do it, at least with long thin pastas...
make a ring with your forefinger and thumb around the bunch of pasta, be sure to dip the tip of your fingernail right into the joint right below your thumbnail. That's an Italian serving for one. Or about 1 inch diameter of long pasta. I usually measure my spiral and shell pastas by the cup. I generally allow 1/4 to 1/3 cup per person.
Alternatively, Google has a nifty calc feature. So, open up google and lets say you want 250 grams of pasta, but measured in cups. Type in "250 grams in ounces" in the search bar which gives you 8.81849049 ounces. Now take that figure and type "8.81849049 ounces in cups"
1.10231131 US cups is your answer. So, just over 1 cup is equal to 250 grams of pasta. ^_^ This calc function works with most all imperial/metric conversions. You can even do maths in it!
http://www.google.co.uk/intl/en/help...tml#calculator