You can easily change the height in the template extra.less with:
Code:
.memberHeader--withBanner .memberHeader-main
{min-height:1000px}
But you'd need to think carefully about how it looks responsively e.g. back to current on smaller screens, e.g.
Code:
@media (min-width: @xf-responsiveNarrow)
{
.memberHeader--withBanner .memberHeader-main
{min-height:1000px}
}