I'm a size 10 UK (44 EUR) shoe and with normal work shoes I bought the 45-46 size of BBB waterflex. This was a tight fit and you have to stretch the fabric fairly aggressively to get them over my shoes, but I would say it was the right choice. You may just be able to squeeze a small cycling shoe into a 44 size overshoe, but be wary. If you have big chunky trainers or walking boots then you'd be looking for at least 2 sizes bigger.
The fabric stretches neatly over a shoe, and there's a nice strong velcro strap underneath to keep them in place. The back zip is strong - a YKK manufacture so it should last a long time. I've had mine for 8 months now and they are showing no signs of wear. Note that I do not walk in them - just take them straight off after arriving at my destination.
The overshoes lose a star because after cycling to work in them (10 minutes in heavy rain) there was a spot of damp on the front of both of my socks about the size of a 10p piece - where the front seam is. Whilst this isn't the end of the world it does mean a damp start to the work day which isn't what I was expecting from an overshoe branded as fully waterproof. This is a flaw in the design and I believe the seams should not be at the front where the fabric flexes and most rain hits - it's probably the case here because you can have symmetrical fabric cut outs and make the overshoe cheaper to manufacture.
The material is fairly thin so there is minimal thermal insulation, but this wasn't a big deal for me - worth bearing in mind though.
Overall, a very lightweight, strong and versatile overshoe, but you might want to buy some waterproofing spray for the front seams if they leak.