When aspiring models walk in on our open calls we just ask them their age and height, measure them if neccesary, take a look at their holidaypictures and in 9 out of 10 cases politely tell them we won't be able to do anything for them
When we recieve pictures by mail or email and we decide to invite the person in question to seem him/her 'in real life' we sit them down, have a chat, take a good look
make some polaroids, do all the measuring, take their contactdetails, do a brief explaination about doing first test, building a book etc, ask if there are any questions and let them know we will get in touch the next day to tell them if we want to work with them.
It's difficult though because sometimes you invite someone because the pictures look promising and when the person walks in you see it's not modelmaterial at all and you feel sorry for making the person come all the way to the agency. We always try to make some time for cases like that too, after all we invites that person all the way up, but it feels a waste of your & the persons time.
Most aspiring models are kind and polite, but some people are very rude when you tell them you wont take them on, or become very persistent
It's no problem to ask why you are rejected, but it is very difficult to explain sometimes because it is much more a 'feel' thing than a rational decision
Oh, and about the height thing. Most agency indeed have girls and guys on their boards that are shorter than their offcial height-recuirements. But it is always very difficult to get those people to work and those models really are exeptions!