Sphere size is the same as the atom's VDW radius.  You can change atom radii with the "vdwdefine" command, or by selecting atoms and bringing up the Selection Inspector (click on the button at the lower right of the Chimera interface) and typing a new value into the "radius:" field.

Ball and stick size are controlled on a per-model basis.  Use the Model Panel (Favorites->Model Panel) and double-click on line with the model you want to control.  A dialog listing the attributes of that model will come up.  The "ball scale:" field controls the size of balls.  It is a fraction that the VDW radius is multiplied by to get the ball size (default: 0.25).  The "stick scale:" field controls stick size and also a fraction that sticks get multiplied by.  The default stick radius is 0.2 angstroms and default stick scale is 1.0, so the final default radius is 0.2 angstroms.  The help page for the attribute dialog describes why there are two numbers involved if you care, and also describes the other attributes if you're interested.

--Eric