Miyata 1400 is an awesome bike. What year is your bike? Looks like Miyata was selling 1400 only for two years. In 1989 the bike had a steel frame, in 1990 - aluminum. They are fairly high end bikes. Second from the top of the Miyata steel bikes lineup (step down from Team Miyata).

Post pictures of your Miyata. I'd love to see it.

Another thing that helps identifying the frame size is looking into manufacturers catalog and finding out what sizes were available for the given model. For example, Miyata's touring models (600, 615, 618 and 1000) were available in 54 and 57cm, but the rest of their road bikes had 56 cm option.

Here's a link to the Miyata catalogs: https://www.ragandbone.ca/Miyata/miyata_selector.html