You can use a combination seven non-exclusive techniques to control bevel radius and sizes either globally or on a per-point basis.
- For simple tubular bevels with a circular section, not using any Bevel Objects, you can directly use the global Bevel Depth radius property, from the Curve tab in the Properties Window to control uniform bevel depth cross all splines.
For curves using Bevel Objects:
- Scale a Bevel Object in Object Mode for uniform scaling across the whole curve. If scaled in Object Mode you can share the same bevel geometry across multiple curves, but each at independent sizes. You can scale independently for each axis.
- Scale the Bevel Object in Edit Mode to affect all curves using a bevel objects sharing that Object Data geometry. It also remains with applied and uniform scale in Object Mode. You can scale independently for each axis.
- Control the Curve Weight on a per vertex basis for precise size control over each point in the curve. In Edit Mode press Alt+S to adjust radius of selected vertex or use the Radius tool. This acts as a percentage factor over all above scaling methods, multiplying its value over the result of Bevel Depth, Bevel Object scale and Taper Object.
- Turn on Proportional Editing by pressing O key while in EditMode to control the vertex weight smoothly along the spline length.
- Use Taper Objects to have another curve object parametrically and non destructively define the radius of the bevel object along the curve. The Taper Object is mapped lengthwise along the X axis of the "tappared curve", using the $Y$ axis coordinates of each vertex on Local Space as multiplication factor for bevel radius.
- Adjust bevel easing progression along vertex from the Radius Interpolation property on a per spline basis. You can choose from several fallof types to affect smoothing, like Linear, Cardinal, BSpline or Ease.
All above methods are multiplied to calculate a final combined scaling factor.
You cannot however control different scales independently on each axis on a per vertex basis. Non uniform bevels scales are only possible by scaling a whole Bevel Object so they are by definition always consistent across the whole curve.
If you require varying non uniform scale you may have to convert (a copy of) your curve object into a mesh and do that destructively afterwards.
Alternatively if you only require a simple tube with circular section, you may also use a mesh object with a Skin Modifier which allows different factors for each axis.