I just replaced the belt on mine and of course forgot to measure it - but I got the replacement from Martin (Dillen).
The old stretched one I have here measures about 45mm in diameter, is square and maybe little less than 2mm thick. I guess you have cleaned and relubed the mechanism, you might not need a new belt after that at all? It was very, very hard to move in mine, and the final culprit was the little pointy thing that clicks the sledge in place at the inward position. I had cleaned everything else but missed that at first - it was sticky and prevented the mechanism from reaching the fully loaded position without help, even with the new belt.