Alan, I got the 1mm holder. What I did was experiment with my scanner a bit before ordering. I tried scanning a sharp negative while propping up the Epson holders on business cards, and also tried it flat on the glass without the holder, and it seemed to get a little sharper when high, so I knew the optimal height for mine would be on the high side. A simple experiment should reveal if yours has optimal focus on the high or low sides of the stock Epson holders, which I think are at about 1 mm.
Concerning the negative strip size, the Betterscanning holder permits me to scan four 6x6 cm square images in one strip, with a fairly thick separation of about 8mm between negatives, and a bit of space left on the end of the strip. The scanner covers the entire length of the betterscanning holder at least.
One last thing. I didn't order any T locks with my betterscanning holder, but now I wish I had. With some films, like Kodak Portra, they are quite flat and don't really need the ANR glass to hold them flat. In such cases, I would rather not have to use the glass, because it's just more surfaces to keep dust free. The T locks would have been sufficient to hold the negatives flat in such cases.