I split the difference in tarp size with a 6×10 foot tarp.
A 6×8 is too small for me (though I spent one memorable night in a blizzard under a 6×8 tarp.) An 8×10 is too wide and needs to be pitched in an A-frame configuration for best results.
A 6×10 can be pitched in a half-pyramid to provide excellent headroom and solid weather protection (until the wind shifts 180 degrees.)

