Embroidery sizing on Ball Caps

Hello again!
I am excited to share that I am getting a new machine in teh next week or two and I will finally be able to stitch out my own caps and beanies instead of having to outsource.
My question is what is the standard size for an image to be put on a cap. It seems to me that it varies, is that because of machine type, cap size? I have had to turn so many jobs away OR send them out (which I hate) becuase I could not do the caps.
Any help would be greatly appreciated. Have a nice day

-Laurie :)