I have the M/L and ride a size 10 Vans Andreas. My size 10 has the same size outer profile as a Celcius Cirrus size 11. So I'm thinking my 10's are longer than the average boot, because Celcius doesn't boast any type of shrink tech.
With my boots, there is plenty of room to go bigger in the M/L Force. The problem you're going to run into with L/Xl is the baseplate being too big for the width of your board if it's 25.5cm or less, it will hang off. I'd recommend going to the shop to size it out, if that's not an option, I'd go with the M/L.