OK Speedracer - all logical and very helpful. I do not have the original tags that (I presume) would have a number that would date the pump so that could be an issue I suppose.
Where do I find the globe cd or the gas pump bible?
Gas Pump Bible:https://checktheoilmagazine.com/store
Here's some suppliers of 15" globes (sorry if I forgot any):https://www.vics66.com/https://www.gaspumpheaven.com/product-category/globes/
KC's Kool Fuel
I am home and I just checked the Gas Pump Bible for you. I happened to be correct, the 177 was manufactured in 1925.
And to answer your original question about dimension, a globe size is measured by the diameter of the lens (or the opening in the globe body).
If you know a brand of gas you would like to use, and like a particular globe from one of suppliers above, feel free to call me (number in my profile) and I can tell you if the globe is appropriate for your time period. I'd be happy to help.
If you decide not to use the round Ethyl plate on the side for your restoration, I would be interested in buying it.