If you have two horizontally split windows and would like to increase the size of the current window, you can do ctrl-w +. A count can be issued beforehand, so to increase the height by 10 lines 10 ctrl-w +. Ctrl-w - will decrease the current window height. To set the current window to an absolute height do N ctrl-w _ where N is the number of lines desired.
In a vertically split scheme, you can use ctrl-w < and ctrl-w > to adjust the width. To set the current window to an absolute width, do N ctrl-w | where N is the number of lines desired.
If you have two windows split horizontally and would like to shift to a vertical scheme, you can do ctrl-w H or ctrl-w L depending on which direction you want the current window to go. Ctrl-w J and ctrl-w K will take a vertically split scheme to a horizontal orientation.