I will openly say if you put an Imperial male next to a Breton male, I will not know which race is which. Nords have their accents and are friggin' huge. I couldn't tell the difference in Morrowind, Oblivion, (untill I got used to the VA.), and Skyrim.
All in all, Bretons should look like fine-featured, petite, pale-skinned, dark and red-haired Imperials. It's lore that Bretons have a noticeable, streamlining curve to their features. They look like Tolkien Elves except far less ethereal and tall, more homely and human.