I agree with others. 3/4" is probably the best overall.
A couple of observations. 5/8" pen blanks represent 2% of a board foot. So, if we were paying for wood, we are paying $50 a board foot at $1.00 per blank. (Your $4.95 blank-3/4 by 6 inches is $158 -edit, $213 on refiguring-a board foot) We are paying for labor-the cost of cutting and the resulting waste. So, the suppliers SHOULD be willing to try to accomodate our needs.
In addition, many of the wood suppliers also supply kits. As the kits require larger blanks, and the price of the kits becomes astronomical-they should realize they are pricing us out of the mass market and into a very "specialty" market. This means, from the suppliers' point of view, the higher our cost, the higher their profit BUT the lower their volume.
Since several of the suppliers are savvy marketers, I think you will see the trend continue to higher profit. Hopefully, however, we will also continue to have lower cost, smaller kits (read 7mm). These make 5/8" blanks useful, since it is less labor for us to reduce to smaller diameter.
So, while I enjoy turning the beautiful woods into large, collector pens, I know I will still sell 7mm at least 50% of the time. This means a market for both. And, if we continue to see new turners entering the market, they certainly should NOT be encouraged to start with a $35.00 kit and a $5.00 blank.
So, like many things in life, the wood blank providers will not be able to live in Utopia. It is a pain to stock more than one size of pen blank, but, if you want to play the game you gotta have short spikes as well as long ones. In the words of that legendary newsman, "That's the way it is..."