personal choice. but the 'norm' is about 1"- 1 1/2"---finishing at either 1/2" or 1"---just depends on the look you like, how wide you want your logs to finish, how big you want the block to be- how many 'rounds' you want to put around each block.
i find log cabin blocks to be horribly tedious- so for me the larger the strips the faster it is going to be over with. that being said- it seems (unless it's a really big-king sized quilt) that you loose the 'look' if the logs are more than about 2".
those of us who make scrappy quilts do not worry about consistant size strips- i prefer log cabin blocks that have variable widths in the logs ranging from 1/2" up to about 2" in one block.