How are you going to water a window box that high?

Use the same size window next to the regular door as you have on the first floor of the house. Same style and manufacturer, too.

A row of windows in the top of the garage doors will add a lot of useful free light.

A window box is not practical above the garage doors. Put it under the new window next to the regular door instead.

I would not do a window box,

Here are more options for your to consider

You could add a vent. Always a good idea if the garage get hot.

You could add a vent and a dovecote for a special look

You could add gable vent and a small window

Did you ask your architect this question?