Quote from RaVaGeThe smallest unit you can work with is 1, it's not that thin, but otherwise you have to use a 3D software and make a model...
You don't HAVE TO make a model for something smaller than 1, but if you're working at this scale it's probably best to use a model anyway.
You can use alt to translate/rescale your object off the grid. Alternatively, you can rescale any brush with ctrl+M if you need more precision.
There's rounding errors involved when you work off the grid (this includes rotations), this may cause invalid brushes and weird faces, you should avoid that as much as possible.
But in this case if you want to make a rope you should just use keyframe_rope instead.