remember there is no such thing as a canvas in xara, like there is in photoshop
setting the page size as acorn suggests will not define your drawing size within xara
assuming you export as a bitmap [PNG, JPG...]:
to export at your set page size, make sure nothing is selected, and then when you export, choose page as the export option on the bitmap size tab of the export dialog:
EDIT - for clarity - the figures in the screen shot are my defaults - yours will be different, pay attention to the DPI, if this is other than the default 96dpi it will alter the size
so you should see this:
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