Revit prevents you from accidentally drawing the element, typically a wall, reference plane, or line, if it is less than 1mm (1/32") on the screen. In some situations, there is also a minimum threshold for moving an element.
Warning: Element is too small on screen.
Solution: If you intend to make the element that small, try zooming in the view. 1/32" (0.79mm) is the smallest line segment you are able to draw. To move an element a very small distance, try using the arrow keys on the keyboard to move the elements horizontally or vertically in small increments (this is also known as 'nudge'). Each press of an arrow key nudges the element a specific distance based on your current zoom factor.