(Updated) As of Androidplot v0.9.7 there are now styleable attrs for controlling the size and positioning of the graph widget from XML.
Keep in mind though that because the graph is a widget component, the sizing / positioning conventions follow those of all other Androidplot widgets. Check out the "x/y positioning widgets" and "sizing widgets" sections of this doc for info on how that works.
Here are the xml attrs currently available:
<attr name="graphHeightSizeLayoutType"/>
<attr name="graphWidthSizeLayoutType"/>
<attr name="graphHeight" format="dimension|float|integer"/>
<attr name="graphWidth" format="dimension|float|integer"/>
<attr name="graphLayoutStyleX"/>
<attr name="graphLayoutStyleY"/>
<attr name="graphPositionX" format="dimension|float|integer"/>
<attr name="graphPositionY" format="dimension|float|integer"/>
<attr name="graphAnchorPosition"/>
So for example, adding the following to your plot's xml element would give your graph a width of 300dp:
ap:graphWidthSizeLayoutType="absolute"
ap:graphWidth="300dp"