Stolt's BootSkis are designed for use with ski boots that support DIN-type bindings. There is no specific information on whether they can be used with crampon-compatible mountaineering boots. Normally, such products are designed more for ski boots that are specifically designed for skiing. If you want to use crampon-compatible mountaineering boots, it would be advisable to check compatibility directly with the manufacturer.
Hello,
I don't have the product at hand...but it seems to me that I had the same question as you when I bought it...for information, it does fit a L size because it's an EU 44/45, and it fits my ski boots! Not yet tested in real use ... Can't wait for the snow!