The universal size is 450 x 250cm.
The Vango Calisto/Asante 400 footprint groundsheet is a durable PE groundsheet cut to the footprint shape of the tent. A footprint groundsheet is useful for any tent (especially if it has a sewn-in groundsheet) as it will keep the groundsheet of your tent clean and mitigate the possibility of a puncture to the main groundsheet by any sharp/hard object concealed on the ground.