Whatever you feel comfortable with really - I tend to decide what I want to say then make it fit.
Using Photoshop I just press Control Button and get the resize (whilst still in edit text mode) and I can fill the area of the template...
What is most important is the border around the outside - this is going to determine the FACE WIDTH i.e. how wide your letters are and how "safe" they are for the cuts coming across them at 90 degrees - ie in my case the Vintage and Old
I have a photoshop template you can have (with the Editable layers still for the two sides) - just message me if you are interested in it