Is that the dumbest question you've read lately? Well, I don't know the answer, and I'm sure it's probably different for every herb.
This minute I am concerned with rosemary. How long is a "sprig" of rosemary.
Say about 12-15cm long. That's about 4-6 inches in your language. Or if you haven't got a tape measure - say about the length of your hand from base to finger tip - you choose which finger! But there's no hard and fast rule about it. You can make it twice as long or half as long if you want!
That's rosemary. You're unlikely to get too many thyme sprigs as long as that. More likely about half the length. It varies according to the age of your plant. Of course, you're likely to use more thyme in any recipe than you would rosemary, so for thyme - use 2 sprigs!!
A sprig of parsley now - that's different. You just cut off the leafy bits at the end of the stalk. Some have lots of leaves, some have just a few. It's a common-sense thing with parsley.
The thing to remember is that precise measurement of herbs isn't required. Add a little, taste, add more if required. If you find you've added to much of the herb, the only way around it to increase the quantities of everything else!!
Here in the south, a "Sprig" is just another word for a cutting.
It's like saying a "Pinch of salt" or "Dash of Vanilla". It's just an inexact amount.
or the same as saying 'too taste'. I like those kind of recipes... course they're just guidelines. :o)
For Rosemary I would say anything that is not yet woody could be cut and called a "Sprig".