I just started using a 6x7, and am finally processing a few rolls of film. I have only experimented with making 8x10 prints so far, which have turned out beautiful. Once I find an image that I am really excited about I will start printing 16x20 or 20x24.
I am not sure what you mean if there is ideal size to print 67 negs. I think it is very subjective and just depends on your own personal vision and how large you want to print the image. The 8x10's from the 67 are gorgeous; incredible resolution, detail, and contrast, with very little effort.
If your concern is about fillling up the entire space of the photo paper, with the image, I really would not be too concerned about that. I print images various sizes, like a 5x7 or 8x10 on 11x14 just to have a a lot of space around the image. The idea for me is to print the image the size where it will translate the best. For example, I have a few 35mm images that I really enjoy viewing, however, they fall short technically, mostly out of focus, and the out of focus is not really pleasing. By printing them 5x7 or 6x9, they image is passable. If I enlarge any more than that they will fall apart.
I am not really sure if I answered your question, but I would just experiment and see what works best for you. It is kind of difficult to know what you are hoping for as an end result.
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