So what is the minimum size for osb outside of a full sheet?
I’ve seen them nearly that small.
They ended up bracing it with a 2x4 toe nailed between the rafters.
This is just stupid, if you step on it its gone.
Thats why they have h-clips. Nor are you going to get a 4x4 or 4x8 sheet to fit perfectly without cutting at some area of the roof.
Atleast they used the h-clips.
Yes you are going to have to make cuts, so, you cut one sheet at 2’6" and put it in the gap then continue on. The cut off can always be used up somewhere else. That method of filling in a 6" piece is just not right.
Larry
The intended function of a pitched roof is not to support someone walking on it. Although we expect it to because most roofs are capabable of supporting us while we walk on them. I have never seen a pitched roof that required the weight of a person to be added in to the live loads.
If you object to the posted pic because other reasons fine.
The OBC and the APA do not have a minimum width for OSB roof sheathing. The only requirements are maximum spacing of supports, either with or without edge supports ( metal H clips or 2 x 2 blocking). Although the carpenters could have done this one a bit differently. There is nothing wrong with the installation.
Although, not in the best standard of practice to in-fill with a narrow strip of OSB, at least they used H-clips and should be fine.
The most loggical installation would have that narrow strip up close to the ridge vent or started the bottom edge with other than a full sheet.
Just a sign of improper planning and execution of the work. See it all the time.
Marcel 
Obviously it would be better to have a larger fill in piece, but it will work. Not good to put it at the peak though. At least this way it has clips on both sides to give it some strength. It’s common for it to bust when put at the peak especially if you are installing ridge vent with ring shank nails. Looks like they lowered it for this reason.
Good insite Steve.
Since the typical ridge vent is located at the peak of the gable and already has a 2"+or minis gap for venting and the ridge cap is six " down each side and the least loading location for live load including someone walking on it, that would be the lodgical place to have a narrow strip. Both are OK.
Marcel