You can simultaneously use four techniques to control bevel radius either globally or on a per-point basis.
- Scale the bevel object directly for uniform scaling along all Bezier curves using that bevel object. You can scale in Edit Mode so the object remains with applies scale, or in object mode if you use the same geometry across different bevel objects.
Control the Curve Weight on a per vertex basis for precise control of size over every point in the curve (In Edit Mode press Alt+S to ajust radius of selected vertex)
Use Proportional Editing (O key) to control the vertex weight smoothly along the spline length
Or use a Taper Object to have another curve interactively and non destructively define the radius of the bevel object along the curve. $Y$ axis coordinates are used as multiplication factor along the lenth.
All above methods are multiplied to calculate a final combined scaling factor.
You cannot however scale independently on each axis, bevel objects is always scaled proportionally on all axis. If you require that you may have to convert a copy of your curve object into a mesh and do that afterwards.
Alternatively if you only require a circular section may use a Mesh object with a Skin modifier which allows different factors for each axis.