In Xara 2's Page Options set your Units to Pixels and under the Grid and Ruler tab, set the Spacing to 100pix (type it exactly like this) and the Number of Subdivisions to 10.
Enable Snap To Grid and draw two squares, to the sizes shown. Disable Snap To Grid. With the large square selected, and the Rectangle Tool selected, click on the Curved Corners icon on the Property Bar (shown in the inset). The corners will round to the default amount which is fine. Select the smaller square and repeat the process. This time, however, drag the corner with the rectangle tool until the rounded area on both squares looks evenly spaced.