I've used my Etowah Outfitters 10x12 rectangle silnylon tarp with my RidgeRunner, hung with a 12' ridgeline. Nice coverage but a bit narrower than the RR's suspension triangles.
As for width and room under the tarp, I opted to insert some cheapo flexible tent poles underneath the tarp and those provided enough interior Width for most activities - even when pitched low - ala 'storm-mode'.
For porchmode I just un-stake one side, insert my hiking poles under the tarp edge, then re-stake the side. Works great! FYI: these are the tent poles from wallyworld's 2-person dome tent, retails about $40, purchased onsale ~$25. No i've never setup the tent
I'll continue experimenting with this and eventually purchase some carbon fiber poles. Then add a small gorssgrain loop with a grommet to the four tarp tieouts where the poles are -to secure the poles into at the tarp's edges. For me - not a distance hiker- the tarp-pole mod is less fiddly than using removable side pullouts provided by several of our HF vendors.
In the end, I'll most likely purchase a cuben tarp to offset other weight... A 13+' ridgeline, in cuben, would be ideal for the RR (and the JRB bridge) and allow a rectangular shaped tarp 's corners to be folded-in like doors.
I'm still a noob; but Hickery taught me the secret to using a tarp's 'doors' (either built-in or folded rectangle's corners).
 suspend the ridgeline,
 clip the two corners together at each end - before setting any side tieouts!
 stake-down those 2 end clips until the rest of your guylines are positioned.
So Simple this caveman had not even thought of it yet! Thanks Hickery!
Not a great photo but shows the poles Not in porchmode (Buck Lake Sunrise, FF3 2013):