it depends on what size finished block you want to make, how many rounds you want and what YOU like.
for me- i generally do not like 2 1/2" strips for logcabins---unless i am making a huge block (18" or larger)
so my strips tend to be 1", 1 1/2", 1 3/4"...once in a while 2"...i just like the design with more rounds and thinner strips-
but it is a personal decision- what YOU like & how big you want your blocks-how many rounds you want-