I would not do that - I would make a vector shape the same size as the base tile [ eg copy the base tile and then intersect it with a larger rectangle]

I would apply the base tile to this vector shape, no need to mess about resizing and risking inaccuracy

I would then build up the larger shape in the way I mentioned before...

EDIT - forgive me you do not need to do the intersect - just do it with the base tile; in xara there is no difference between a bitmap filled shape and a bitmap