The Size Modifier scales the mesh. Different scaling factors can be applied along the x, y and z axes. The Size operator only changes the vertex position and does not affect the local coordinate system, i.e. the reference point for other modifiers, such as the velocity field, are not changed. 

Name: Provide a name for the modifier that closely describes its function.

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Lock: When enabled, this flag keeps all the scaling factors the same. The last change in any axis is adopted for all axes.

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x,y,z: The scaling factor by axis.

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Anchor: The anchor is the xyz position in space around which the scaling will be applied. This may shift the whole object towards or away from this position, depending on whether the value is smaller or larger than one. A change in anchor position does not affect the local coordinate system, it only moves the vertices of the mesh.