The latest ORR figures give lot of detail, but only at macro GWR level, not split by local trains, regional, West Country, commuter etc.
Per table 4.1 ran 10.5m passenger Km Jan-March (4th highest behind GTR, Northern, Scotrail) which is 96% of 2019 (pre covid)
Table 5.1 shows 69.5m vehicle Km Jan-Mar, 93% of 2019 (second biggest, behind GTR)
Simple maths shows trains average 6.62 vehicles (69.5/10.5) , whereas in 2019 was 6.83 vehicles, so basically on average trains are shorter.
Table 2.2 has the passenger Km, which are 88% of pre-covid Jan-Mar 2019, but journeys are 15% longer.
Comparing the fleet with same period in 2019, it is basically same, except lost some Castle HSTs, a few 143s were replaced almost one for one by some 150s. Slight reduction in 158 fleet due to accident, and number of 387s has changed. Clearly the 5car and 9car IETs haven't been shortened as units, so the average 0.21 vehicles per train reduction has to be running higher proportion of shorter trains.
As 2nd biggest train operator (by vehicle Km in table 5.1) it is worrying that unable to secure extra stock, and it's trains have got smaller.