here's a handy little thing I saw in the book Log Cabins and Cottages : William S. Wicks originally published in 1929

I got my copy off half.com for under $5.00

I am sure some of ya'll are familiar withe this but it was new to me and you know I like to share when I find cool stuff.

anyway in an illustration of recommended log tools was this simple guage that you could make from plywood. obviously my mspaint drawing is not to scale but you get the idea. Its a quick reference for log selection that will tell you the diameter of the logs at a glance, just stick this guage around your log until it stops. be sure to mark the openings for how big they are, heck you could even make two, one for small and one for big, graduated by an inch.