Did you know that the size of a particular bolt is usually indicated in the Suzuki Part Number for that bolt?
I need to replace the pillion bolts with mushroom head as I will be mounting a larger tire and it might rub on the stock bolts.  I did not know the size of the bolts, but an online fiche showed a part number of 01550-0816A.  
The first 4 digits after the dash give you the bolt size and length.
8mm bolt, 16mm long.  So, I did not have to pull the bolt to find out what size mushroom bolt I would need.
This is USUALLY correct on short bolts.  
Not so much on the longer bolts (over 99mm long)

This, and other fun Part Number info I found here: https://forum.suzukiclubuk.co.uk/viewtopic.php?t=6351