At the National level, the BSA has no restrictions on blade sizes. Extract from the Guide to Safe Scouting:
"Knives"
"A sharp pocketknife with a can opener on it is an invaluable backcountry tool.
Keep it clean, sharp, and handy. The BSA believes choosing the right equipment
for the job at hand is the best answer to the question of what specific knife should
be used. We are aware that many councils or camps may have limits on the type
or style of knife that should be used. The BSA neither encourages nor bans fixed-
blade knives nor do we set a limit on blade length. Since its inception, the BSA
has relied heavily on an outdoor program to achieve its objectives. This program
meets more of the purposes of Scouting than any other single feature. We believe
we have a duty to instill in our members, youth and adult, the knowledge of how
to use, handle, and store legally owned knives with the highest concern for safety
and responsibility.
Remember—knives are not allowed on school premises, nor can they be taken
aboard commercial aircraft." (page 48)
Link:
https://filestore.scouting.org/filestore/pdf/34416.pdf
As mentioned, there are local councils, camps, troops and event organizers that prohibit sheath knives, blades of a certain length, etc. If so, it will be mentioned in the leaders' guide for the event/camp/etc.