Sorry I can't help with the bolt size, but I have to give a word of warning.
I bought second hand roof bars for a 405 years ago, also without the bolts.
I bought bolts from B&Q, the thread was fine, but it was too long, I though
the resistance i was feeling was because the thread might have some rubbish
in it, so i turned a bit harder and snapped the bolt in the hole. Ended up
having to have a bigger hole and thread made for that corner. If you can't
find bolts short enough, just hack-saw off longer ones,

Neil