Reading the article On the asteroid belt’s orbital and size distribution (Gladman et al. 2009), there is an interest point made:
asteroids a few kilometers and smaller in size are recent collisional shards of larger objects since their lifetime against collisional destruction is much less than the age of the Solar System. As such, these bodies preserve information about collisional breakup, the dominant physical process that has shaped the present asteroid belt.
This suggests that there would have been a lot more asteroids etc in the main belt at the time prior to the Late Heavy Bombardment (~4 billion years ago) - which is after the major planets had formed.
My question is what was the greatest spatial extent of the Asteroid Belt in the period just before the Late Heavy Bombardment?
(I am after peer reviewed resources).