If you print of the probolt catalogue its got the bolt sizes in like diagrams you can put the bolts againts to tell you what length and type, catalogue is about 48 pages though. I printed it out at work lol
The other way to check sizes is to use your allen keys, these should have a size marked on them, just compare the bolts to the flats on the keys. But IIRC Shimano v brake bolts are an odd design.
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