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 differently as the photo itself has a value and the buyer can be found later. So, when we look at different types of modeling we are looking at different classes of clients. As an example, the fashion industry has become very dependent on using models in its advertising, promotion, and sales material.
So let consider the job market when considering types of modeling projects and if you can make a living as a model. In a large marketplace like New York there is an enough work in a given category of modeling that a model can specialize. A model can be just a fashion editorial model or fashion catalog model or even a plus size model and be able to make a living. In smaller markets one would have to be more versatile to make it. When look at jobs and who can make it as a model I like to view it as a pyramid. The pyramid represents all of the jobs available for a given year and the shape is formed by the requirement for the jobs. The large base is made up of standard fashion jobs, commercial modeling jobs, and the large numbers of other job where they want someone who "looks like a model". Only models who fill the Standard Fashion model requirement (tall, thin, and beautiful) will fill these jobs.
As we move up the pyramid and the job pool gets smaller we get to the Plus Size Beautiful models and the Petite Beautiful model. There are fewer fashion jobs available but one can still find commercial jobs (you are of course competing with standard size models for these jobs but size is less important) . As you move further up the pyramid to the Plus Petite Beautiful models and the Older models, the job possibilities get smaller still. An at the top are the Special Beauty, "Real People", and Special Character models. At this top of the pyramid are the jobs that come up once in a blue moon, but are great for modeling schools and model searches to give as examples when giving their pitch that anyone can be a model.