How to set size of part of plot control where graph is displayed?

How should i set size of plot without axes? i know properties Size but it modifies the whole control, but i need to set size of paper where graph displayed (without axis etc). I need X-axis width 500 pixels. Or explain me how to calculate what i want from control's size please.

WPF: you can set the width of the plot control and the margins of the plot area in the plot model. Also set AutoAdjustPlotMargins to false!