No portion of the tarp should extend beyond the perimeter of the tent. If it does it will wick water between the tarp and tent bottom resulting in wet tent. I custom cut a tarp for our tent that is six inches smaller all around than our tent bottom.... it's made of left over tyvek house wrap. Works great.
Tent camping hints:
1. Dont' buy inflatable mattresses. They inevitably tear/lose air overnight/are a PITA. Spend your money on some good 'thermarest' style camping pads. You'll sleep better. Or buy solid foam pads or egg crate.
2. NEVER WEAR SHOES IN YOUR TENT. They just track in dirt/mud/mess. If you have a covered 'vestibule' shoes should stay outside. If not, then shoes should be left in a box or bag by the door.
3. Tent mosquito netting should ALWAYS stay closed so you don't have to spend the night with those little buggers.
4. I was glad that I 'seam-sealed' all the seams on my tent.
5. PRACTICE putting your tent up with your camping partners. Practice in the dark, holding your flashlight in your mouth or in the light of your headlights.
6. After camping make sure your tent is COMPLETELY dry before putting it away. The google earth picture of my old house has my tent, fully erected in my driveway, drying out after a camping trip.
HAVE FUN.
. We had the foam mattresses (we called them "Rubber B****es"). Never had a restless night. On the weekends I would camp in the Olympic Mountains with a roll up backpack model. Sleep on my sweater for a pillow. 
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