When we get a call we ask for address and immediately go to zillow.com for gross sq ft of lot. Alot of times I will just go with that (thats being realy lazy) but it has worked out well. We also use findlotsize.com. I ask them a few questions over the phone and then I give them a price while emailing the estimate.
If they accept than after asking a few more questions I tell them all i need is a CC# to get them on the schedule. Done!