There is no set rule of which bit to use to make a sign. It all depends on the finished product you seek. Your question is wide open. What material will you use? What thickness? Will it be painted, sanded, shaped, bent, laminated, etc.? What font? What size? What depth? Recessed or protruding text? Chamfered? Rounded edges? Hollow (requiring some stencil configuration)?
Sign making is an art all its own. The letters can be any configuration you desire, as long as you have the tools and the ability to achieve the final product.
I don't understand why you would 3D print a template for lettering though. That seems like an awful lot of work unless you are going to make hundreds of signs with the exact same templates.
Can you post an example of the type of sign you want to make?