Rear window rubber - small window vs large window

[Nevada51]

So I ordered a rear window rubber for my '64 small window C-10 but was given a large window rubber instead. Normally you'd send it back but I bought it about two years ago, so that's not an option. My question is: can I use the large rubber and trim it to size, or do I need to buy a small window rubber? Doesn't look like there's any corners on it, so I thought it might be ok to just trim.

If it is ok to trim it, does anyone have a good measurement of how big it should be?

Appreciate your thoughts on this.