Scaled Down Plates
Deciding what size a plate should be is a relatively simple matter when following a full sized prototype. All you need are the dimensions of the orginal (say 15" long and 7.5" in height) and then divide that by the scale your miniature version is built to.
For instance, if the scale used was 4" to the foot, that's one third scale, requiring a plate 5" long by 2.5" wide. This will naturally produce a plate that has exactly the same proportions as on the full size version.
Relevant link - Scale Plates
Freelance Plates Not Following a Prototype
Scale plates are relatively simple to deal with in terms of size and proportion, but what do you do if your model is freelance, or you are adding a plate that the original didn't have (e.g. an owner's plate) and you have nothing to work with in terms of what size a plate should be?
The thing to do is to simply cut up some paper or card to represent the 'footprint' or entire surface of the plate. Sticking that in your intended position on the model, using something like 'Blu-Tack' will quickly give you an idea of what looks right and what doesn't.
If it looks right, it is right.....
Take your time on this, and don't worry if early attempts don't look right. It is actually as productive to get a template to look wrong as it is to get it right, as at least you'll know what does not suit, and all it will have cost you is a little scrap paper or card. Stand back and look at the whole picture from all angles and maybe sleep on it, then do the same again just to be sure. Many will say that "Beauty is in the Eye of the Beholder", and that's true, but proportion is also entirely in the eye of the beholder, and that's you. In general, folk have a good idea of what looks right in terms of proportion, and if it looks right, it probably is right, that's what proportion is all about.
Once you are happy with your paper template, measure it up using in Imperial or Metric mesurement, let me know what those measurements are and I'll do the rest.