F-Class bags have been largely adopted from benchrest, but you can repurpose a sandbag if you want (no rules there, so long as it does not attach to the rifle). Several factors to consider - how high, how hard, how you hold the rifle, how heavy, etc. Some broad generalities here, but a free recoil type of hold will normally require a heavier bag than if you are pressing the rifle into your shoulder. Bags do seem to be getting bigger and higher, but depending on your flexibility, a higher bag may be required to ease your neck. You may need to reconsider the size of your bag if you are flying and having to deal with weight limits. There is still a rule on the books that requires the bag to be deformable by finger (but I have never seen it checked at any level of competition .. and the bags just keep on getting 'firmer'). Last - you'll sometimes see regional difference in gear, largely due to who locally may be bringing in specific products, so don't be disappointed if no one locally doesn't use your favourite brand.
Best to drop by a match and see what competitors are using and ask some questions. The group at CHAS are quite helpful and not too far from you.