I've never tried on Alegria in a store, so I was relying on my experience with Dankso and Sanita to guess the right size.  I normally wear an 11/11.5 depending on the brand.  In Sanita that equates to a solid 43.  As I am wearing them with compression stockings, they still feel a bit big - but the need to have plenty of room in the toe box - makes me think that sticking with 43 is the correct thing.  I have been walking about the house and have noted that if you have the strap too tight - they are uncomfortable, but correctly loosening it makes walking in them like walking on clouds.

I think that choosing the right size is the biggest hurdle.  If you are on the smaller end of a size I would order your normal size, but if you are on the top end of a size - do yourself a favor and go up one size.  This style needs for you to have plenty of length in the toe box to enable your heel to glide up and down like most clogs.  I don't believe they are intended to be form fitting like a pair of tennis shoes.

Lastly, about the arch.  My foot is pretty flat with almost no arch - Dansko is painful for me to wear.  As you make your selection, keep the arch in mind - if you have high arches and require a lot of support - this style is not for you.