Secondary FPV camera decisions will be size, image definition (TVL), and perhaps field of view (FOV).
FPV Camera Size
Many of the popular CCD FPV cameras used today are about 1.25 inches square (30mm or so). They are also often called 1/3″ cameras because they use the very popular 1/3″ Sony CCD chip.
They can be encased in a plastic or metal case for protection or have the PCB board totally exposed if you want to save a little weight at the expense of protection.
The encased ones weight about 30 grams and will have several different resolution capabilities. Again there are many different cameras out there, but these little square 1/3″ CCD ones are the most popular right now. Most also allow you to thread different 12mm mount lenses into the lens body tube to change your field of view or even increase the light gathering capabilities with a lower f stop number lens (much like changing lenses on an SLR camera), making them very adaptable.