tri cycling shoe fit?

hey, i just recently bought my first pair of tri cycling shoes.  they are exustars and are carbon (were like $100 off on nashbar).  I was just curious as to how these or biking shoes in general are supposed to fit.  With these new shoes, there is a lot of slip and my heel feels like its going to slip out of the shoe when i start going hard, these are a size 44 (i already sent back a pair of 45's and got these).

  I usually wear a 10.5 (45) in my old Shimanos.  While they feel like they fit good and there is no slip after about 25 miles or so my feet begin to go numb.  I thought maybe becasue they are too small? 

 I'm just not sure whether my heal is supposed to be sliding around so much, it feels very inefficient, although having my feet go numb also doesn't seem right.  So i'm just curious as to how cylcing shoes are supposed to fit. 

 thanks,

darrin