A vital sizing key for a BCD is the amount of lift the BCD is able to generate when inflated. Depending on the amount of "kit" you wear, the amount of lift required will differ. You can always use a larger BCD (with more lift) in warmer climates, but not necessarily the other way around.
Here is a simple guideline for you:
- Tropical Diving (shorty or only 3mm wet suit)
- UK Recreational diving (5mm wet suit, dry suit)
- Technical diving (dry suit, twin sets, demanding diving)
8 to 14 kg
10 to 22 kg
There are also many different inflation mechanisms. Manufacturers have been trying to differentiate themselves through some of these features - e.g. Mares Air Trim; Aqualung i3 and the Scubapro Balanced Power Inflator (BPI). Personally, the traditional Inflator hose, which has been around for a long time, works very well and when we teach, we use it - to ensure everyone uses the same simple equipment.
Some BCDs offer an alternative air source at the end of the inflator hose, effectively diminishing the need for an octopus. However, this means you will need to get it serviced regularly (a normal BCD can easily be serviced by yourself if you follow the instructions - we recommend you get shown by an equipment specialist on how to service yours). The advantage is that it reduces your travelling weight, but they are quite bulky and do require a bit of practice to use them effectively.