In Tinkercad, you can resize an object in any direction you like by dragging its corners. Unfortunately, you’ll also disrupt its proportions (your perfect circle is now an oval).By holding the shift key as you drag, the object will hold its proportions as it scales up and down in the direction you pull it.
Alternately, by holding both shift and the alt key together while you drag, the object will proportionally scale in all directions, regardless of which direction you drag.
Finally, holding the shift+alt key combination while dragging an object’s top handle will proportionally scale the object in every direction from it’s center.
Here's one of our own TinkerTip videos to help showcase the different options you have while scaling objects.