We have been selling the Thule Adapter Pack with AFK for many years and are confident the medium AFK is the best size for this purpose.
The measurement of the 2.5m Thule is the full awning measurement - The Thule 2.5 measurement is including the whole casing. The internal measurement is smaller than 2.5 which is why we use the medium AFK.
The kit compatibility is below.
This kit is designed to be compatible with Thule Omnistor Side Mount Awnings including the 4900 and 5200 range with a 2.5m projection. It is not compatible with roof mount awnings such as the 6200, 6900, 9200, or 5102 range. It is also not compatible with awnings with a projection larger than 2.5m including the 8000 range which has a 2.75m extension.
If you have concerns about the compatibility with your caravan awning I would contact the dealer and confirm the awning is side mounted and is one of the ones listed as compatible in the advert.
But in terms of our kit - if you have a compatible awning the Medium AFK is the correct size for the installation.