What mason jar size is best

You can cut and preserve wood slabs pretty easily if you're handy, there's lots of good articles online for doing so.

For centerpieces, I'd go at least pint, but you need to size them up appropriately with the wood. A tiny jar in the middle of a huge wood slab might look funny.

Agree with PP it really depends on other aspects of the centerpieces I'd start with the wood

If you are having a reception outside you need to have something to weigh the mason jars down. I set up the centerpieces for my FSIL wedding. It was windy and the flowers make the mason jar top heavy. I put water in the mason jars and it wasn't heavy enough. I got a couple of table cloths all wet. I was collecting rocks and putting them in the mason jars so they would not blow over. I was able to conceal the rocks because the mason jars were painted.