Sounds like a solid foundation to your sleep system--light and versatile. Still need to select a ground cloth and pad/mattress and you don't want to scrimp there, either.

As to the tarp, I'd tend to go with the longer option, as in bad weather you'll be pitching it closer to the ground and the extra sheltered space will be valuable, especially with wind-driven rain.

Don't have any particular tarp pitch guides in mind but know they are out there. Will suggest a lot of practice pitching in different settings and circumstances at home before heading out--you don't want to be puzzling over an odd pitch when it's raining and the sun is going down. (Not that that ever happened to me, nope, no way....) The biggest delineator is probably below vs. above treeline. If you don't hike with trekking poles you need to improvise support.

Happy experimenting.