Dave, it sounds like you have a painted turtle or red ear slider or similar species. They are most common in that size range you described. The box turtle shell I use is 4" by 5" I have seen them smaller and bigger, but not by much.
You can make a call out of the shell you have. You just have to have more calling surface material and it's more difficult, because the bottom and the top part of the shell are attached together.
A box turtle shell Is what most people use, it's smaller and the shell is deeper. Also, the bottom and top of the shell are held together with meat. So when people find them the bottom part is usually gone and your left with just the top.
I'm not even sure if you have box turtles in Canada, some states don't allow you to even have possession of a shell. You might wanna do some research on that.