Your unfinished block sizes will be:
2.75 inches with just 4 patch,
3.682 inches after first triangles added,
5 inches after second triangles,
6.864 inches after third triangles, and
9.5 inches after fourth triangles.
I know this because the one and only time I made snail's trail blocks, I made a spreadsheet to compute all the required triangle and square sizes, as well as the size of the unfinished (and finished) blocks along the way. (It's kind of hard to square up to 3.682 inches, isn't it?)
After doing the math, and struggling with bias edges, and trying to line up those triangles ever so carefully, I decided to paper piece the blocks instead. I found this was much easier and faster and I was very happy with the result.