For all standard Mac windows:

1. Drag the bottom right corner to the desired size.

2. If the bottom right corner is off the screen, move the window by the top edge (where you see the window title) to bring it back.

3. If the window is too close to the top of the screen for this to be possible, unlikely if your window is "small" but possible in other situations, click the green circle (the one with the plus sign that becomes visible when you mouse over it, third from the left, if your color vision is impaired) at the top left of the window. That will resize it to the screen and make the bottom right corner visible.

4. If you use multiple monitors, move a window entirely to the external one, and disconnect that monitor without moving the window back, it's possible for a window to disappear entirely. Fear not, nothing has gone away; you just can't see it. Post again if you ever run into that problem.