All the sizes listed should fit any 550, 5-series (225, 335, 445, 555) or 7-series 107, 207, 307, 307L) bag. The exception being some of the taller ones won't fit in the smaller (less tall) sized bags like the 225.
You should choose which Partition you want based on the lens/gear that you intend to put in it. For example with a lens that is 10cm wide and 18cm long:
- Would be a perfect fit in a '10-18' size
- Could be squeezed into a '9-18' if you wanted (it would just push the sides out slightly and create a tighter fit).
- An '8-18' would probably be too tight.
- An '10-15' would be too short - the lens would peak over the top.
- A '10-21' would be an option if you didn't want the lens to reach all the way to the top but certainly wouldn't fit the shorter (less tall) bags such as a 225.
To help you decode the size names - on a '10-18' size:
- '10' refers to the width - 10cm wide. It also refers to the 'square' side of the Superflex Partition - 10cm x 10cm (when viewed from above). A 10cm x 10cm area would be idea for a lens with diameter or 10cm or less in this example.
- 18 refers to the height - 18cm.
- The depth (front-to-back) is always 15cm regardless of the size name.
Also remember to use the diagram above to understand the sizes. The grey images are a 'top-down' view. The white images are the view along the shortest side - so you can understand the height. Clicking the different size names at the top will also show you a diagram of that particular size about a 3rd of the way down the page.
P.S. 1cm = 10mm = approximately 0.39 inches.