According to this author, red blood cells (RBCs) are biconcave to allow easier bending. The standard explanation in biology for higher surface area to volume ratios is that it improves reaction rates. When a RBC squeezes through narrow veins I imagine that it gets deformed. Considering that a RBC's function is to exchange oxygen in narrow capillaries, I was wondering whether it is appropriate to reason that the biconcave shape promotes gas exchange. So what is the SA:V of a biconcave RBC and by how much does the SA:V change depending on where in the body the RBC is traveling?
I googled "surface area of a red blood cell" and this paper was the first result. It should answer your first question. Though it was unclear to me what the surface area unit of measure was, which may require a bit further reading. There's also this article, which goes a lot more into the math of calculating these two things.
As for your second question about how much it changes, intuitively I would guess very little and it probably does not significantly change exchange rates. I couldn't find any research on the topic. The hard part is defining all the shapes they may get squeezed into, then either calculating or measuring SA:V. Possible? Surely, a well written computer model could do it at some point. But worth the time/money? Probably not.
human erythrocyte avg dia: 7.2, vol 91.31 cubic microns, surface area 136.55 square microns