Of course, if you don't pull the corners all the way to the ground, there will be more space inside.
I'm 6'4". My 104" long, 57" wide Sea-to-Summit poncho tarp works fine as a lean-to and fairly well as a modified lean-to with one end staked down and the other end open. I've slept under this poncho tarp and another similar one with a lower version of this pitch, faced into some protection and away from the prevailing wind, through some fairly rainy nights, and managed pretty well by wrapping my ground sheet up over the outside edge of my bag and accepting a little dampness from splatter and blown rain that I mostly dried out during sunny periods the next day. This would NOT cut it for a week of prolonged rain. The trip the photo below is from was Hell's Canyon, on the Oregon, side and in typical high desert fashion would alternate between hot and cold and rainy and sunny throughout the day. For this, the poncho tarp was perfect.
Modified Poncho Leanto.jpg
It does not give me enough length if I try and close off both ends. And, a 1/2 pyramid is completely silly and unworkable for me. Heck, a 1/2 pyramid with an 8x10 tarp is only marginal at best for my length.
The closest I have ever come to sleeping in a 1/2 pyramid pitch, even with an 8 x 10 tarp, is the following pitch, and I still have to sleep diagonally and scoot my feet down to the limit of the back corner to keep my head adequately inside the shelter. If I had to batten this 8 x 10 tarp pitch down further to a full 1/2 pyramid and still sleep inside, I would have to put my feet inside my pack liner to keep them dry as they would sit right against the outside edge of the tarp.
semi-half-pyramid tarp pitch.jpg