How can I determine what size my watch face is in an 8 I bought and don’t have box info with me

Read the response you received from Carol B., which advises you to look on the back of the watch, which will say what size the watch is. And as noted, if you can't read the writing, take a photo of the back and then zoom with your fingers on your iPhone to be able to read it.

And if you truly can't do that, open the Watch App on your iPhone. Tap My Watch lower left corner. Tap General > About > Model (tap Model twice to reveal the model starting with a Letter, followed by 4 numbers) and then post the model number and we can tell what size it is from there.

Look at the back of the watch face. It is listed there. It’s hard to read so I took a picture of it with my iPhone then enlarged it to read it more easily.