To toggle between a full screen editor and a nearly full screen terminal you can use:
{
"key": "ctrl+alt+m",
"command": "workbench.action.toggleMaximizedPanel"
}
with your keybinding of choice to replace the Ctrl–Alt–m : that is mine. You just need to “maximize” the terminal first – pull it up as far as it goes. It will remember that between sessions.
Revisiting this :
As of v1.38 this is now pretty simple. There are two commands that will toggle the panel/editors to full screen.
Pick some keybinding to use for the toggle trigger:
{
"key": "ctrl+alt+q",
"command": "workbench.action.toggleMaximizedPanel",
// "command": "workbench.action.toggleEditorVisibility" either one
"when": "!terminalFocus"
},
The above will expand the panel or editor to full height, but toggling back will return the panel to its original size but not to nothing. If you want the terminal to bounce between full open and full closed try both of these keybindings:
{
"key": "ctrl+alt+t", // you could use "key": "ctrl+`", if you wish
"command": "workbench.action.closePanel",
// "when": "terminalFocus"
},
{
"key": "ctrl+alt+t",
"command": "workbench.action.toggleMaximizedPanel",
"when": "!terminalFocus"
},
The order of the above 2 keybindings is important.

v1.50 is adding the setting panel.opensMaximized – I tried its different options but couldn’t get a simpler result than the two keybindings ctrl+alt+t version I showed above. In any case, start with the panel closed for it to work well.