FWIW - I don't have a (22 inch barrel) 'Philippine Constabulary or School rifle', nor, do I have a (26 inch barrel) Board of Ordnance & Fortifications rifle to measure.

However, it is possible to extrapolate the necessary stock length of a Krag for a given barrel length and allowing for acceptance of a bayonet.

Officially made - Springfield Armory Krags display three barrel lengths:

22 inches - Krag Carbine models and Philippine Constabulary & School Rifles adapted for a bayonet.
(School Rifle & PC Rifle stocks would measure 38 inches - OAL).

26 inches - Ultra-rare, Board of Ordnance & Fortifications rifles.
(BoO&F Rifle stocks would measure 42 inches - OAL).

30 inches - regular Krag Rifles.
(Regular U.S. Krag Rifle stocks measure 46 inches - OAL).

A non-official Krag Rifle, with its barrel shortened to 24 inches, would require a stock approximately 40 inches long, if adapted to accept a bayonet