MVT opens to match the size of your browser window. To make the MVT window larger (or smaller), first resize your browser window: notice that MVT stays the same size. Press the "Reload" or "Refresh" button at the top of your browser: MVT will resize to the new browser-window size, without loosing the tools (plots, numerical answers) open in MVT.
Running MVT as an application on your personal computer
If you can't type in MVT's text fields, then you must click on something in another window other than the browser (i.e. click a link in another browser window) and then come back to this window containing the MVT.