650px corresponds to the portrait mode in smartphone.
900 px to the landscape mode in a smartphone, so you can decide if its better to choose 650 or 900px for your case.
Or use the XF syntax
Less:
@media (max-width: @xf-responsiveWide)
{
/* CSS for Wide Width Screens Here */
}
@media (max-width: @xf-responsiveMedium)
{
/* CSS for Medium Width Screens Here */
}
@media (max-width: @xf-responsiveNarrow)
{
/* CSS for Narrow Width Screens Here */
}