Resizing the windows at the edges/corners is really difficult in Ubuntu MATE, as the trigger area is only one pixel wide.
As a workaround I edited /usr/share/themes/Ambiant-MATE/metacity-1/metacity-theme-1.xml and increased bottom_width, left_width and right_width from 1 to 4, but this also visually enlarges the window borders, so that might not be everyone’s cup of tea.
It would be really useful to have either some invisible trigger area just outside the window borders (maybe over the shadows) or at least include a theme with the above changes.