My interpretation of The MSA Yearbook 2018 is 1.75 litre AFFF as the minimum capacity for vehicles prior to 1st January 2019.
The relevant text is in (T) Trials, Car Trials Formula, Safety (page 358), paragraph 11.7.3., which states 'Fire extinguishers to safety criteria K.3.1.1'. Paragraph 3.1.1. states 'Small, Hand operated'. But what is 'small'?
The definition of small appears to be in (K) Competitors: Safety (page 174), paragraph 14.1.4., which states 'Fire Extinguishers. Even small extinguishers carried in a vehicle can extinguish or contain fires before they develop seriously. Minimum recommendation is for a 1.75 litre AFFF extinguisher or equivalent with BS4123/EN3 approval (EN3 minimum size is 2 litre AFFF) and a rating of at least 34B. More sophisticated equipment is required in many events and full vehicle systems are highly recommended'.
Competitors: Safety (K), Appendix 3 (page 195), which also mentions capacities, is relevant to 'Recommended for all vehicles. Mandatory for new build vehicles from 1st January 2019 and all vehicles from 1st January 2022.'
If I've got that incorrect, somebody please correct me.