How big a model is determined by the scale ratio. For example A real Boeing 737-800 is about 129 feet long, if the model of the 737-800 is 1/200 scale it would be the original size divided by the scale or 129 feet divided by 200 which equals 7 3/4 inches long. Scale ratio is expressed either 1/200 or 1:200. The smaller the scale ratio the larger the model will be.
For Example here are the approximate measurements for a 737-800 model by scale
|1/44||=||2 feet 11.18 inches|
|1/72||=||1 feet 9.5 inches|
|1/100||=||1 feet 3.48 inches|
Most model airliners are 1/200, 1/400 or 1/500 Scale. 1/200 Scale are great desktop models, they normally will be big enough to display and are ideal for offices. 1/400 or 1/500 Scale are great for model airports the planes are small and you can fit more planes in a smaller amount of space.
Fighters and miltary planes are much smaller than airliners and therefore fighter models should be scale 1/72 or larger to make for an ideal desktop display.