It's not possible to say to the inch as it depends how you angle the awning poles but loosely:
A 4 has a 4' body and the beds are just under 4' so 12' by just over 6'
The 4/5 13' and the 5 14'
Don't know what you were going to buy for a groundsheet but as a cheepy I use Visqueen [You must be registered and logged in to see this link.]
Buy from a builders merchant NOT B&Q or it wont be cheap. I tend to use as a double thickness.
You will get at least two seasons from it and if it does become laden with mud to the point you don't want to mop it's cheap enough to throw. The roll width is 4m so if you buy a 4m length it should cost about £10/£12.
Alas though colour co ordinated for the blue and white Dandys it clashes a bit with brown and the black Visqueen is very black.
Not exactly grass friendly but those groundsheets aren't waterproof and just help tramp up mud in this weather.
Buy the thick tarp type solid PVC sheet if you want don't buy a cheap woven tarp. They are the worst of all worlds, not grass friendly and not waterproof.