Dear Xara...

Ensure Snap to Objects is active.
Press Alt-I-S-E or Shift+Ctrl+Alt+E to create a Smart Ellipse.
Drag the ellipse to the centre point of the page.

Without switching to the Selector Tool (Alt+S), make the ellipse a 317px diameter Circle.
How easy was that?

I used Shift+Right Drag to keep the ellipse in the middle of the page but for the first horizontal axis, I had to change Zoom to 10000% to get down to 0.1px accuracy.
Luckily, after changing Zoom back to the Drawing, I could hit 317px spot-on for the vertical axis.

Now, Clone and increase the size of the Circle so that its vertical diameter aligns with the left edge of the original and its bottom does not change its vertical height or the right is horizontal position.
Basically I am asking for you to double the diameter.
How easy was that?

I rotated the circle 180 degrees and then used the Resize control.
You cannot judge the doubling accurately as nothing Snaps: no guide, no shape, no grid.
If you work at it the Resize pop-up will indicate 200%, let the drag go free and restart and it is reset to 100%.

No matter, you got there.

Well, not really.
Switch to the Selector Tool.
The original "circle", for me, is 317.9px (W) by 316.1px. Over 0.25% inaccurate.
The "doubled circle" is 635.8px (W) by 632.2px (H). At least the Resize indicator proves to be accurate. Sadly, it is also off-centre.

Go Old School.
Ellipse Tool (L).
Drag out an Ellipse, size and align.
Clone and Scale 200% or drag and Snap at the edge of the original or even input Wx2 with Hx2.
Check centres, dimensions, orientation, perimeter and area. All show up a mostly circular. Accurate to 0.015%. Probably better if I had used milliPoints.

The final nail is when you Ungroup a Smart Ellipse, you end up with a Shape not an Ellipse.

For aficionados, a Rounded Rectangle is the better shape to use for an Ellipse for web purposes as its HTML representation is a SPAN (310 Bytes) and not an image (8 kiloBytes):
<span class="xr_ar" style="left:1096px;top:677px;width:200px;height:13 7px;background: -webkit-linear-gradient(180deg, #FFFC00 -0%, #FF0000 100%);background: -o-linear-gradient(180deg, #FFFC00 -0%, #FF0000 100%);background: linear-gradient(270deg, #FFFC00 -0%, #FF0000 100%); border-radius: 100px/68px;"></span>

The Good The Bad and the Eggly.xar

Acorn