I'm doing a wedding cake the bride wants 2 layers each on a 10", 8", & 6", with a C initial on the top. What size of initial would you recommend? 2"? 4"? or larger?

It depends on what you want it to look like? If you have just the C then you might want at least a 4in. If there are flowers you might want to go with something bigger so it wil show up.