What size objects can be created from collision of two asteroids?
Since planets are forming from asteroid collisions, I would say those collisions won't necessarily create small objects.

Imagine throwing a snowball against a window.

If you throw it lightly, some snow will stick to the window and increase the mass of it (at least until it melts away) and the rest of the snowball will be scattered everywhere around but won't stick to the window.

If you throw the snowball too hard, the window will break, resulting in even more tiny pieces. But still, some snow will stick to some pieces of glass.

That's exactly what happens on asteroid impacts and depends on the speed, mass and structure of each asteroid.

The size of those debris should reach from microscopic dust up to gigantic rocks with several kilometer wide diameter.