In my experience you get "splash" only if you don't want headroom, and you also need to be more careful how you pitch the smaller ones. I've been out with my litle 5x8 siltarp but I needed to be very careful of the pitch and keep it fairly low
in order to have the coverage enough to avoid side splash and blow in. however it worked fine even in a good storm.

an 8x10 or 7x9 gives you acres of room, and you can make sure you have lots of headroom. (and yes a hammock makes it even better ) but by far my favorite tarp for ground dwelling is the integral designs silwing - light, cat cut, and just the right size for a compromise between "enormous" and "big enough that I have tons of room and can avoid weather". I've now done west coast trail
three times using that as a shelter. (well, I do also throw in a nano-7 hammock, and I have been known to hammock a night or two as well). the silwing is a bit small for a hammock tarp (it's not really long enough) but works with the little bitty nano-7.