You can use the ‘ArrowStyle’ property to set the arrow head-size in points, the half-angle (use angles >90° to make inward-pointing heads) and fill pattern (you must give this as a number). The fill pattern is solid (=15) by default, and the default half-angle is 25°.

ch.Set 'astyle'  6 30

These settings are ignored by arrows on axes, which are always solid filled and scaled relative to the axis line-weight. Examples include FrostFree and the wind-rose in the Polar examples.

See also ...
Shading Patterns
Style – chart style
XStyle – X-axis style