Actually the bolts are M10 by 95mm, but they are somewhat special in that they have M10 fine/medium threads (1.25mm) & 14mm across flats hex. The locking nuts are also 14mm spanner size.
The hex head at the top, inside the riser, does need to be 14mm as it locates in a matching recess to prevent the head turning when tightening.
Also, when I fitted them to my Tbird yoke, I found I needed to add a washer at the yoke, as the TBS risers are an oval shape & don't quite fit in the circular riser recesses on the Tbird top yoke.
The stock TBS bolts also have a drill hole for fitting an 'R' clip. These Triumph parts are long enough to add the washer at the top if the clip is ommitted. Which is fine imo, as the stock nuts are steel locking type, not nylon.
M8 bolts, say 100mm & trimmed a little, if required, could be used but std. hex is 13mm (I think?). You would want to check if 13mm would still be captive in the riser. This is a useful feature because when fitting it enables the bars to be fitted & tightened in proper alignment before tightening the riser nuts.
I'm using M8 high tensile bolts with TBS risers on my Kawa top yoke, with nylock nuts, which I'm happy are strong enough (& I use high bars) but in your situation I'd probably buy the Triumph bolts & matching nuts for ease of fitting.
I have these TBS bolts & nuts, but one of the bolts is slightly bent, otherwise I'd offer them to you. (Didn't notice when I originally fitted them!)
98 Tbird tank/seat, Sprint fairing, ZX7R forks+Racetech Gold Valves, 08 ZX6R shock, Alcon 6-pot, Ignitech+TPS, T'bike 3-1+Beowolf can, airbox mods/rejet, tubeless wheels, lots :) per mile.