How large could gravitational waves get and what effect could they have on us?

Wave strain scales as $1/r$; the waves that were detected came from a source about a Gpc away and had a strain of $10^{-20}$.

To get a strain that would affect things at the 1% level, you would need to be $10^{18}$ times closer, or at about 30,000 km from the merging black holes.

The tidal force due to the black holes is roughly $GMmh/4r^3$, where $h$ is your height, $m$ your mass and $Msim 60M_{odot}$ the combined mass of the black holes. Assuming 1.8m height and 80kg mass, the tidal force stretching you is around 50N. So I think you would notice this more than the gravitational wave, and even more so if you were to get any closer...