In the '50s, '60s, and '70s a female model had to be about 5' 9" and a certain dress size. Men were something like 6 foot and a 40 regular. Of course you had to be beautiful.

The reason for this was that fashion manufacturers made dress and suit samples in one size and it was easier to find a model in that size than to make a sample in a different size. As the plus sizes and petite sizes arrived and super models of varying sizes came on the scene, these old rules had to change some.
At the same time the "fashion look" for a season changed as to what is considered beautiful. One season the look is anorectic drug addict, the next innocent and medieval. I don't think the classic beauty look has been in since the middle eighties. In the secondary fashion markets, the look that might be needed can vary even more. When it comes to commercial modeling, size, look, and age can vary wildly. In a secondary market the more you can vary your look rather than having "a" look, the better your chances of finding work.

So, ultimately, it all comes down to three things:

1) Having the look that someone needs(generally the client) for the project they are working on at that time,

2) Making life easier for the art director and the photographer so they will want to work with you, and

3) Making sure the client, photographer, and art director know you exist and want to work.

The first has a lot to do with what you inherited genetically (your looks and talent) and what you have done with them. The second and third are where being a professional model comes in - knowing what to do and how to market yourself. When a project calls for someone attractive to stand next to a new product, the person who can constantly look good in front of the camera, show the emotional expression that is needed on cue, show up on time and leave on time and in so many other ways make the shoot go quickly, efficiently, and successfully, is the person who will get the job. That is also the person who gets asked back the next time. The part after the genetics is what I consider makes a professional model.

Guide lines for what it takes to be a model

Well, all the above is fine but does not give much to go on, so do you have what it takes to be a model? The first thing you have to consider is what type of model do you want be? If you are trying for high fashion modeling, the runways of Paris, the cover of Vogue, etc., the requirements are stricter. If you are hoping to do commercial modeling (such as product or lifestyle) there are much broader requirements. If you want to be a glamour, or pin-up model, it has its own set of requirements.

First Thing to Understand

All modeling, except for some glamour, is client driven. There is no need for a model (or for that matter, a photographer) until a client has something to sell (a product, service, or idea). At this point, the client becomes willing to part with the dollars to accomplish his/her sales goal and a model is hired. It is also the size of the potential sales' goals and the model's importance in those goals that determines how much a model is paid. Some types of glamour and stock lifestyle modeling can work differ...