If you want to change, you can use 16 count and it will be the same size exactly. You just take the stitch count listed on the pattern, which usually has them listed as ##W x ##H.
You can do other sizes in the same way, dividing any stitch count by the fabric count. I always used to confuse myself trying to recall how to do this. Now, it is easier because I recall Stitch Count ÷ Fabric Count = Size of Design for me:
San ÷ Francisco = San Diego.
I feel entitled as I lived both places.

You can certainly use another fabric count. The only difference is that Marilyn often charted her faces over one thread on the 32 count fabric, and the rest of the design over two, so there would be more detail in the face. I believe she included an option for Aida fabrics as well either with the chart or on her Told in a Garden website ...so you could stitch the face. You just wouldn't get the same detail.

I love this pattern! I have been looking at it too, and recently added it to my wishlist. I already have the Angel of Autumn pattern, and haven't tackled it because it looked so intimidating to me! Please share your progress; I'm curious to see how a fabric change affects it.

Miss Kitty, you have me worried now... I am looking at my Angel of Autumn pattern, and I don't see any indication that the face is stitched over 1, instead of 2. How would I know if that's how it's supposed to be? Surely it would say so on the pattern. So, in the absence of special instructions for her face, hopefully it's safe to assume it's all done over 2. I looked at the website, and didn't find any clarification there.

If you have a Lavender & Lace angel chart that does the face and/or hands 'one over one" there should be a small close-up insert cross-stitch graph showing a close-up of the face (and/or hands) on the same side of the pattern of the angel, that you follow to stitch those areas.

Thanks, Bermuda. That's good, that means it can just as easily be stitched on 16 ct Aida.