... 5x8 rectangular tarp. What I'm wondering is how you get out of your wet clothes and into bed WHILE it's raining. Under a small tarp there's not a lot of room to get out of your wet clothes and into your sleeping bag w/o bringing in a lot of water with you.
I won't say it's easy. I find that I sometimes have to fight off a bad attitude and tell myself that this too will pass. Therefore I won't sugarcoat the experience.
I have used a 5x8 tarp a fair amount, an MYOG beaked TarpTent 1 from Henry's plans more often, which isn't much larger than 5x8. Also a tarp based on Jay Hamm's plans from here on BPL, which is a bit shorter than 8 ft but beaked on both ends and is wider on the head end even after making the head end 16 inches narrower than his plans.
Keep in mind that I am neither small nor limber.
Tarp get's setup first. For hard or windblown rain it's pitched pretty low, not necessarily high enough to sit up in ... but I will sit up anyway while getting situated.
Pack goes under the tarp, the bottom end extends beyond the drip line.
Then a groundcloth (30"x80") or bivvy (adapted from SMD Meteor plans) are deployed under the tarp. They have corner guylines that run to the tarp's corner stakes ... so they are held in place.
I go in next and sit on the groundcloth or end of the bivvy (the head end unzips pretty far and the top is pushed toward the foot end). My head might contact the tarp, big deal? No.
A quilt, kookabay pillow, balaclava and pad (neoair) are in the same silnylon stuff sack. Pull that from the pack and set it out of the way towards the foot end.
Shoes come off and are placed next to groundcloth or bivvy, still under the tarp. They make a good bin for things I will want during the night(like meds, headlamp, small water container). Raingear comes off and is rolled up tight and set somewhere under the tarp. Similar with other wet clothing. The groundcloth or bivvy are getting damp where I am sitting, the pad will cover that soon.
Neoair comes out of the sack and is inflated and positioned under me ... resulting in about 1.5 inches less headroom (get used to it).
Inflate pillow. Get inside bivvy if using that. Get settled.
Sooner or later I'll want insulation. Quilt comes out of the sack and is deployed on my lower half. Balaclava set in a dry place. The sack goes inside out and wet clothing goes inside that ... minimizing loose gear. Finish putting on quilt and balaclava.
More or less reverse the process in the morning if it is still raining.